The 3 times Yuusaki Riko appears in the Anime..
Sweet Feelings are a Taste of Chocolate
Motto To LOVE Ru
Saruyama has a daydream about Yuusaki Riko giving him Chocolates and a kiss.
You are the Lovely Cinderella
Motto To LOVE Ru
Saruyama falls in love with "Riko-chan" and asks Lala to invite her for a date; out of pity, she forcefully changes Yuuki Rito into "Yuusaki Riko" so Saruyama can go on a date with "Riko-Chan". After that Yuusaki Riko only appears 3 times in the anime and in the manga for short amount of times on rare occasions.
Rito becomes a woman
To LOVE Ru OVA Episode 1
From Wikipedia
Yuusaki Riko is Yuuki Rito's female alter-ego, she is a beautiful girl (with a physical appearance resembling a tomboy) who gets much attention from men, such as Taizou Motemitsu, the principal and even Kenichi Saruyama, Rito's best friend.
"Rito, Becomes a Woman"
"Rito, Onna ni Naru" (リト、女になる)
To LOVE Ru OVA 1
When Lala heard that Rito and Saruyama are talking about breasts
she decided to make an invention that will make her breasts appealing to Rito.
then All of a sudden Rito crashes into Pai Pai Rocket Kun which explodes on him turning him into a girl that he gave the name Yuusaki Riko.
