Miyu Kōzuki is an 8th-grade student whose parents leave for the United States to work for NASA and arrange for her to stay with their long-time family friend, Hōsho Saiyonji, a monk who lives at an old temple atop a hill. However, the monk himself leaves soon after for India on a year-long voyage, leaving Miyu in having to stay together in the same house with his son, Kanata.
Suddenly, the landing of a UFO which transports an alien baby, Ruu, along with his cat-like "sitterpet", Wannyā arrive after they were separated from their own planet, Otto, due to falling into an interplanetary worm hole, a time warp which can transport objects across several regions, planets and timeplanes. They cannot return to their home planet due to it being too difficult to reach, and Wannyā therefore requests Kanata and Miyu that they allow them to stay in their house. They agree. Miyu and Kanata learn from Wannyā that people from Planet Otto look identical to human beings, and that he himself can transform into human beings, animals and objects.
Miyu and Kanata feel a very deep love for Baby Ruu, who also cares for them deeply and likens them to his parents due to their similar looks, and often go to great lengths to help and protect him and Wannyā and as the story progresses, the group are often involved in comedic and funny situations but grow to greatly love each other. The story ends with a rescue team from outer space coming to Earth and returning Ruu and Wannyā to Otto safely and Miyu going to a boarding school alone. Though years later Miyu and Kanata reunite, get married and have a daughter named Miu. They also reunite with Wannya and Ruu!
Miyu Kōzuki (光月未夢, Kōzuki Miyu)?, (Seiyū - Kaori Nazuka)
* Birthday: March 15th
* Blood Type: O
The main female character in the story is Miyu, a blonde and good-natured girl, whose parents leave for the United States to work for NASA at the beginning of the series, arranging for her to stay with their long-time family friend, Hosho Saionji, a monk who lives at an old temple on a hill but the monk leaves soon after for India for a year, leaving Miyu and his son, Kanata liveing together by themselves.
Miyu often fought with Kanata at the beginning of the series, but finds out from her mother that she was childhood friends with him at a very young age, and starts to like him more and more and develop feeling for Kanata as the story progresses(which Christine gets all hyper). At first Miyu always deniding her won feeling but soon, she began to realized and accept her feeling for Kanata.
In the manga, it's shown that Miyu and Kanata has kissed for a few times. In Volume 9 (Chapter 45), Miyu is shown that she wanted to tell Kanata her real feeling toward him but failed.
She also had a slight crush for Mizuki Yamamura, Mikan's younger brother because in one of the episodes when she and Kanata help Mikan in her manga there was a character which Miyu admired, she says that that is her ideal kind of guy, when she went to Mikan's apartment she met him, Mizuki Yamamura and had liked him because he is exactly like the guy from Mikan's manga. But in the manga, Hanakomachi Kurita (Momoka's older brother) has shown a crush toward Miyu and made her feels uneasy about it.
She feels a deep affection for Ruu, who likens her to his mother due to their similar looks. Miyu is shows as an ordinary girl with a kind and generous like a real mother, she is a full of smiles and cheerful girl but also have a soft heart that's easy to be touched.
In episode "Nozomu-kun Goes to the Dream World", it shown that Miyu's most important dream is to be together with her family, including Kanata's family (Mr Hoshou & Mrs Hitomi), Ruu's family and Wanya. It seems like she is very caring towards Christine, which she showed when it was the episode "The Hike of Fear", where she said to Wannya not to hurt her.
It revealed in Chappter 41 that Miyu was born after Kanata and they had become friend since then. Miyu's name was originally from 'Mi' - Miki and 'Yu' - from Yume which meant 'dream'.
At the end of the story, she goes to a boarding school all by herself. After school she and Kanata meets again and gets married and had a daughter, Miu. They also get reunited with Wanya and Ruu too!
Kanata Saionji (西遠寺彷徨, Saionji Kanata)?, (Seiyū - Yuko Sanpei)
* Birthday: December 25th
* Blood Type: AB
The main male character in the story is Kanata, who is among the most popular boys in his school, especially the girls, due to his good looks. Shortly after Miyu arrives at his home, Kanata's father declares that he wishes to leave on a year long trip to India for enlightenment, thus binding him and Miyu to living together in the same house.
Miyu starts attending Kanata's school and they have difficulty keeping their secret about living together. He finds out from Miyu's mother that they were both childhood friends and starts to like her more and more and also starts to develop feelings for Miyu as the story progresses (but still teases her to mask it). His mother, Hitomi was the good friend of Miyu's mother (Miki Kazouki), died while he was about 3 years old.
Kanata never want to admit his real feeling toward others, especially Miyu. He cares for his loved one very much, such as his family, friends, Wanya, Ruu and Miyu. Kanata does have feelings for Miyu as he said in episode "The Weird Object That Seiya Left Behind".
Kanata's favorite food is pumpkin, and likes all its variants, from squash to the chips varieties. In episode "The Blackout Holloween", it's revealed that Kanata had like pumpkin ever since he was a baby when his mother feed it to him for the first time. Ruu also calls Kanata "papa" because he looks very similar to his real father.
Kanata is shown as an easy-going person and doesn't like to talk very much unlike Miyu. Even if he seems a quiet and a bit cold attitude, Kanata is actaully a very loving and caring person to everyone he loved. Even thought there are many romantic between him and Miyu, he often makes a joke out of it to cover his real emotion and feeling.
Since he was little, Kanata loved his mother very much even thought he only had meet her for a short time. In the manga (Chapter 41), as Miyu and the others went back in time, Kanata went to the time where his mother was still alive. Miyu who was wearing a yukata (given by Kanata) had the chance to meet Kanata's mother and they both where wearing the same yukata.
When Miyu goes to boarding school in the end, he is separated from her but reunites with and marries her years later. Afterwards, they have a daughter, Miu. They also reunite with Ruu and Wannya.
Ruu (ルゥ, Rū)?, (Seiyū - Mika Kanai)
Ruu is an alien baby from the Planet Otto, who can levitate and perform telekinesis on objects, and who develops a deep love for Miyu and Kanata, likening them to his parents and often assisting and helping them. He uses his abilities at very inappropriate times causing Miyu and Kanata great trouble.
Although he is still only a baby, he can understand everyone perfectly. Ruu loved Miyu and Kanata as his own parnt very much, he loved them very dearly and doesn't like ti when Miyu and Kanata fight like in episode "With Ruu-kun at The Department Store".
In that episode, Ruu manage to make peace between his two beloved 'earth' parent. Ruu is also shown as an ordinary baby that can cries and laugh like normal babies do.
In order to keep the others from knowing the truth about Ruu's identity, he is referred to as Miyu's little brother although Ruu will deny that and insist on calling Miyu "mama". So he believes Miyu and Kanata are his real parents but does not learn the truth until the end of the series.
Wannyā (ワンニャー, Wannyā)?, (Seiyū - Chiemi Chiba)
Wannyā, also known as Baumiao (which is a play on "Bowmeow"), is a cat-like alien "sitterpet" from Planet Otto who is entrusted to take care of Ruu and can transform into all sorts of human beings, often a blond-haired man who was once the subject of Mikan's affection and a dark red-haired woman, and all forms of animals and objects. He develops a passionate liking for mitarashi dango and often writes about the events of the progressing day in his computerized "sitterpet" diary. Wannyā often does all the cooking and cleaning in the house.
Christine Hanakomachi (花小町クリスティーヌ, Hanakomachi Kurisutīnu)?, (Seiyū - Haruna Ikezawa)
* Birthday: September 2nd
* Blood Type: AB
Often nicknamed just Chris, Christine is Miyu and Kanata's pink-haired, half-French (via her mother) and insanely wealthy classmate. She is so deeply infatuated with Kanata and becomies extremely jealous whenever she sees him with Miyu. She often expresses her rage by demonstrating her superhuman strength by picking up large objects, such as trees and desks, to throw in the process. The moment she snaps out of one of her "episodes", she will apologize sweetly and fix whatever items she has damaged in her rampage, with tools kept in hammerspace (i.e. construction equipment). Her incredible jealousy is usually meant to be comical, but is sometimes used in serious situations. Chris appears completely oblivious to her supernatural powers but she appears to be timid and friendly. Coming from a very, very rich family, among her possessions are a winter lodge and a snowy mountain. Christine's parents are not seen in the anime; instead, her companion is a butler named Shikada ("Mr. Deerfield" in the English translation) who is dressed up as a deer. He has been doing this for Chris since her childhood in order to make her happy, due to her deep love for deers. Her favorite pastimes include skiing and horse riding.
Momoka Hanakomachi (花小町ももか, Hanakomachi Momoka)?, (Seiyū - Chinami Nishimura)
* Birthday: June 21st
* Blood Type: O
Christine's 3-year-old cousin. Momoka often drives along in her tiny toy tricycle and befriends Ruu, declaring that he is her boyfriend and that they are in a close relationship. Momoka has an odd grudge against Miyu, often calling her Madam or obā-chan, a title used for an elderly woman or grandmother, which in result infuriates Miyu all the time. Momoka learns Miyu and Kanata's secret about having two aliens in their house in the episode "Momoka's Passport", where she saw Ruu fly in the sky in order to get the ballon.
Aya Konishi (小西綾, Konishi Aya)?, (Seiyū - Atsuko Enomoto)
* Birthday: August 24th
* Blood Type: B
Aya is Miyu and Kanata's classmate who is a very good friend of Miyu and often goes shopping with her and Nanami. She has a greenish hair color that is styled in two braids. She likes school plays, which she often directs. Much to Miyu and Kanata's displeasure, some of Aya's plays or projects are extremely similar to their current life, they get annoyed sometimes when she makes Miyu and Kanata the lead roles in the play, and that is much worse when Aya develops an interest to Ruu and often features him in the plays. Aya, on occasion, has been known to start speaking French whenever she is excited.
Nanami Tenchi (天地ななみ, Tenchi Nanami)?, (Seiyū - Asako Shirakura)
Nanami is Miyu and Kanata's classmate. She has short brunet hair and is a very good friend of Miyu and often goes shopping with her and Aya. Nanami is usually very playful and energetic but can sometimes be easily tired after a hard task. She also has a huge appetite. She likes old school style of clothes.
Santa Kurosu (黒須三太, Kurosu Santa)?, (Seiyū - Hikari Tokida)
* Birthday February 2nd
* Blood Type: B
Santa is Kanata's best friend, who often comes up with strange ideas. He has a deep love and appreciation for old records. Miyu once thought that Santa had fallen in love with a girl, but later finds out that it was instead the girl's camera he had fallen in love with. He has a penpal, whom he develops a liking to and vice versa. Unfortunately, she left for Germany, but not before the two meet each other in person before her departure. His name is a play on the Japanese pronunciation of "Santa Claus", though this has no direct relation to his character and may be named after the capital of New Mexico, Santa Fe.
Nozomu Hikarigaoka (Hikarigaoka Nozomu)
* Birthday: August 8th
* Blood Type: B
According to himself, he is Kanata's rival. Nozomu is a classmate of Miyu and Kanata with blonde hair. He has a pet bird named Okame who helps him in distributing his roses to the girls. He debuts in the first episode of the second season because he was continuously stalking Miyu which makes her think that he was an alien only to find out later that Nozomu was interested in Kanata.
Mikan Yamamura (山村みかん, Yamamura Mikan)?, (Seiyū - Mika Kanai)
A budding mangaka (manga artist), Mikan often has a tangerine on her head(same goes for the rest of her relatives, even her pet Scottish fold cat) and is a very good friend of Kanata and Miyu, both of whom she often helps out and who have also helped her once with her manga. She has a handsome young brother, Mizuki Yamamura, who was the inspiration of one of the characters in her manga, and often helps her out. Mikan is often panicking because of her deadline and often comes to Kanata's house for consolation, having a few close calls to finding out Miyu and Kanata's secret. Mizuki pointed out that Mikan never gave up on her dream on becoming a manga artist, despite her father's wishes on her becoming a baker. So far, Mikan has shown to be a quite popular with her career. Although Mikan looks plain, stressed, even comical most of the time, she is actually very good-looking when with makeup or when she's relaxed. Her name is a play on that of the manga's author, Mika Kawamura. Interestingly, Mikan means " Tangerine" and Yama means "mountain" although the word Yamabuki means " orange-yellow".
Miki Kōzuki (光月未来, Kōzuki Miki)?, (Seiyū - Yuko Minaguchi)
* Birthday: July 7th
* Blood Type: A
Miki is Miyu's astronaut mother who is obsessed with anything connected to outer space and is hired by NASA, along with her husband, Yu, at the beginning of the story, thus having to leave for the United States, arranging her daughter, Miyu, to live with the Saionjis. Her two childhood dreams were to have a beautiful daughter and to become an astronaut, both of which came true. She was friends with Kanata's mom ever since she was a teenager.
Yū Kōzuki (光月優, Kōzuki Yū)?, (Seiyū - Takuma Suzuki)
* Birthday: April 4th
* Blood Type: O
Yū is Miyu's astrophysicist father who is helping his wife with her work, and is hired by NASA, along with his wife, Miki, thus having to leave for the United States, arranging his daughter, Miyu, to live with the Saionjis. He became interested in astrophysics in order to further his wife's dream in becoming an astronaut and traveling in space.
Hōshō Saionji (西遠寺宝晶, Saionji Hōshō)?, (Seiyū - Kenichi Ogata)
* Birthday: June 16th
* Blood Type: B
Kanata's father, who is a monk and the patron of the Saionji household. He goes on a pilgrimage to India soon after Miyu arrives at the Saionji household.
Hitome Saionji (西遠寺瞳, Saionji Hitome)?, (Seiyū - Sakiko Tamagawa)
Kanata's late mother, who was a very good friend of Miyu's mother, Miki, and passed away when Kanata was still a child. Her remains are buried on the Saionji Temple grounds.
Miss Mizuno (水野先生, Mizuno-sensei)?, (Seiyū - Katsuya Endō)
Miyu and Kanata's teacher, who is talented in numerous skills, ranging from cooking to ninja training.
Seiya Yaboshi (夜星星矢, Yaboshi Seiya)?, (Seiyū - Asami Sanada)
Seiya is a blonde-haired alien who visits Earth and takes a liking to Miyu, often disguising as Kanata to get close to her. His kind can read minds through a touch of the hand. He has a sister, Rui, who saved him once from being sucked into a worm hole, and has had a sheltered childhood. His surname, Yaboshi, means night star.
Rui Yaboshi (夜星流, Yaboshi Rui)?, (Seiyū - Junko Noda)
Rui is Seiya's older sister, who greatly cares for Seiya, saving him once from being sucked into a worm hole.
The Principal (校長, Kōchō)?, (Seiyū - Naoki Bandō)
The eccentric principal of Miyu and Kanata's school, who is extremely fond of monkeys, bananas and video games, especially monkey-based ones, his fascination with the creatures stemming from his deep friendship with his monkey, Monkichi (モン吉?, seiyū: Orine Fukushima), from whom he was disunited with in his childhood due to the monkey having been sucked into a worm hole, but with whom he is later reunited, thanks to the help of Miyu, Kanata, Lou and Wannya. All forms of monkey dolls and replicas adorn his office, where he is often found playing video games, and has sometimes even set the themes of school meets to monkey-based ones.
Pepo (ペポ, Pepo?)
An alien rabbit that becomes Ruu's pet after an accident falling from a truck from Outer Space later in the series. In the manga, he is all white and debuts late in volume 7.
Akira Kijyou (Kijyou Akira)
* Birthday: September 28th
* Blood Type: AB
Akira is Kanata's childhood friend who appears in episodes 33 to 34. She has long black hair and her eyes are sort of purple. Kanata made a promise to her that they would watch the sunset together at Fantasy Park which was made true when she visited him because his father was on a business trip. Akira has a crush on Kanata which was seen in their conversation in the Ferris Wheel. She said to herself that "there is no one I like in America" when Kanata told her that the sunsets must be beautiful in America.
In the manga series Kijyou wanted to see what Miyu feels for Kanata by giving Kanata a fake kiss.
Kurita Hanakomachi (Hanakomachi Kurita)
* Birthday: June 21st
* Blood Type: O
Debuts only in the manga. Kurita is Momoka's older brother who falls in love with Miyu at first sight. Kurita looks like a geek or a nerd. Kawamura Mika, the author, describes him as "because he is always wearing those plain glasses, he seemes to be plain yet innocent boy". Kanata describes him as someone who expresses his feelings obviously. When Kurita's glasses were taken off accidentally, Miyu sees that he is a handsome boy.