ユウ

A calm and a very soft-spoken girl who suddenly appears and vanishes, leaving Tsukasa at a loss. Because she speaks in a very quiet voice, people have a hard time understanding what she's saying.

Ai is an AI which is a clone of her.
Her creator is Futakami Kyouko, Tsukasa's mother. Her first name is Tomodachi Ichigo / Friend No. 1, this name come before she can "talk" or "see" the outside world. Her second name is Alpha, this name comes after she can "talk" and "see" the outside world for the first time. Later the name of Yuu comes from Tsukasa's idea and her wish to be named by him.