Yuuichi is a member of the Fuuka Academy Student Council. He first appears in Episode One on the same ferry that Mai Tokiha is taking and the two quickly form bad impressions of each other. Shiho Munakata calls him her older brother even though they are not related by blood. He works with the Student Council doing odd jobs for them (prompting Mai to label him a "lackey") and sits next to Mai in class.
Yuuichi used to be a member of the Fuuka kendo club and regularly made it into the nationals until he was suffered a severe injury to his knee at some point in the past, though the exact circumstances and nature of the injury are never explained in the anime. Masashi Takeda of the kendo club is seen looking sadly at Yuuichi's name card on the wall of the kendo dojo at one point during the anime. Disenchanted, after his injury Yuuchi became a feared delinquent, but Shiho manages to convince him give up his antisocial ways.
Despite the external mutual dislike, it is clear that Yuuichi has feelings for Mai. While they did not get along in the beginning, their attitudes towards each other softened somewhat after episode 5, when Mai breaks down and is comforted by Yuuichi. He happens to witness Mai as she is about to kiss Reito Kanzaki and tries to halt the budding relationship, much to Shiho's distress. After Shiho is attacked by the SEARRS Orphan he becomes suspicious about the recent events at the school and tries to convince Mai to tell him what's happening.
After the insecure and possessive Shiho realizes that Mai and Yuuichi are developing feelings for each other, she becomes mentally unbalanced and makes Yuuichi kiss her in front of Mai, playing on Yuuichi's guilt that he was unable to prevent her from being hurt. He becomes the Most Important Person for both Mai and Shiho and ends up paying for this when Shiho's Child dies at Mikoto's hands, resulting in his own death (ironically, he had realizes this and urged Mai to destroy Yatagarasu herself; when Mai couldn't do it, the maddened Mikoto dealt the killing blow). Just before he disappears, Mai confesses her feelings for him and tries to kiss him.
Eventually he is revived by Mashiro Kazahana and comes to Mai's defense using Miyu's discarded sword when the possessed Reito attempts to attack her. Tate swears to protect Mai and shouts that he loves her, much to Mai's relief. Despite this, at the end of the series both of them are still being claimed by both Reito and Mikoto, and Shiho respectively.
In the manga, Yuuichi is the protagonist and the one who is the transfer student rather than Mai. He finds out that he is the Key to both Mai and Natsuki, two well-known rivals at the school, exasperating and intensifying their mutual antipathy. Because of his dual Key status, he is often referred to as a 'Perverted Beast' by his classmates. His relationship with Mai is much like the one in the anime, although there are further complications. i.e., Natsuki.
Yuuichi was forcefully dismissed from his former school's kendo club after he attempted to stop the sexual assault of Shiho, costing his team the championships because of their disqualification. During the assault, one of the roughnecks pulled a knife out, and gave him a long gash proceeding from his elbow down to his wrist on his left arm. He is extremely sensitive about it, and it is the reason he refuses to join Fuka's club despite team captain Takeda's insistence.
Yuuichi does not explicitly exist in the anime as he is never mentioned by name, however silhouettes resembling him come up during discussion of Mai's past. Mai was supposedly in love with this man, according to Natsuki's version of events, and was torn by her duty to her country as an Otome, but a DVD special reveals this not to be the case.
This individual asked Mai to be his family's Otome, but Mai was conflicted by her choice of where to go after graduation, and eventually wound up becoming the Otome to the Cat Goddess Mikoto after she fell into the Black Valley.
It had been speculated that 'Yuuichi' is actually the young Sergay Wang due to having the same character designs, although this was never confirmed and the two do not share the same voice actor, while all other characters first seen in My-HiME had the same seiyu. Even thoguh he lost his memories after he got shot by Nagi.
There are not one but two Yuuichis in the My-Otome manga, as is with almost every character from the My-HiME manga.
The first is the same Yuuichi as the one in the My-HiME manga. He appears in flashbacks experienced by the HiME Mai, who sees him as having played cruelly with her feelings. Mai also pejoratively claims that he was "that kind of man" who seduced her pitiful self.
This Yuuichi eventually appears in person to Manshiro when the latter is trapped in Fuuka Academy by Mashiro. He exhorts Manshiro to realise his true identity and provides the prince with the Pure White Diamond and the Sword of Akatsuki, enabling him to escape.
Together Manshiro and Yuuichi wield the Sword of Akatsuki, first deflecting a shot from Kagutsuchi and then fatally impaling Mai on it. After Manshiro kills Mai, she returns to her original benevolent personality and is reunited with Yuuichi, who asks Manshiro to win for "their" sakes before disappearing for good with Mai and Mikoto.
The second is the former Emperor of Windbloom and the father of Mashiro and Manshiro. His only appearance is in Chapter 42, in a photograph kept by Mashiro. No further information exists within the manga, making him an enigmatic figure.