Kotegawa Yū

Yuu Kotegawa is Yui's older brother, and a ladies' man, which she dislikes about him. However at the moment, he is dating Akiho, Haruna Sairenji's older sister, but unknown to him she isn't taking the relationship as seriously as he does.[14]

Appearance
Yuu is a tall and fit young man who has light brown spiky hair and gray eyes.

Personality
Yuu is quite calm and easy going. He is something of a ladies' man, but he can also be very protective of women when he sees them in trouble. He enjoys teasing his sister about her relationship with Rito, like mentioning him in front of her and their parents.

Plot
After finding Rito and Yui being chased by hoodlums, he defends the two of them from the ruffians, while noticing Yui's interaction with a boy like Rito and commenting that he hopes Yui will grow to be more mature.[13] Since then, he has taken notice of Yui's relationship with Rito. He can easily read his sister's poorly hidden feelings and her love for Rito, and is eager to see his sister become more open to romance through Rito. Every now and then, most of his appearance would show him teasing his sister about stuff like going on a date with Rito.

Yui Kotegawa
Yui is his little sister. She greatly dislikes how he can be a ladies' man and how he always brings up Rito in front of her, to which she occasionally overreacts and blushes, something he enjoys teasing her about, he is aware that she has feelings for Rito and finds her very easy to read. He can also be very supportive of her some times. Yuu hopes that she will be more mature and accept her love for Rito.

Rito Yuuki
Yuu first noticed Rito when he saw him trying to defend Yui, and has since been interested in keeping track of his sister's relationship with Rito. He would often mention his name at home in front of Yui just to make fun of her.

After speaking with each other (and sharing a beating from Yami), he understands more about Rito's life with aliens and all the misfortunes happening around him. He sympathizes with Rito, but remarks that at least his life is more exciting. Rito seems to like Yuu, as he finds him to be a really cool and understandable person.

He is one of the few people who knows that Riko Yuusaki is in fact Rito.

Akiho Sairenji
Akiho is someone Yuu is currently dating, but she isn't taking it as seriously as he does. He feels disapointed everytime she turns down his invitations with some excuse.