Yuu Shapiro

Yuu is Isabella's younger cousin, who is polite, kind, and cares for the small things in life. While he isn't a fan of the Sonic series he dose have a Digimon partner named Damemon.

Personality
Before Yuu came to the Digital World, he is quiet, compassionate, and above all emphatic to all life. He cares greatly for even the smallest forms of life, like a butterfly, and hates bringing anyone to harm. However, when DarkKnightmon opens a portal to the Digital World for Yuu and claims to be taking him to a world where nothing really dies or gets hurt, Yuu feels that he can act without remorse, as he isn't really hurting anything. Because the Digital World isn't alive to him, he doesn't show any empathy to either digital creatures or fellow humans he meets, believing that no one gets hurt in the end even if he were to kill them. Because he thinks there are no consequences, he treats the Digital World like a game and acts without regard to the safety of anyone in it. When he realizes that the Digital World contains living creatures capable of dying, he feels intense remorse and regret for his actions.

Yuu is an excellent strategist, able to come up with good battle actions on the spot, something which even DarkKnightmon praises him for.

Description
When Phineas confronts Isabella in the Forest Zone and demands to know why she is working with the evil DarkKnightmon, she reveals that she is looking for Yuu, and will do anything to save him, even "following the devil himself." For his own part, DarkKnightmon had promised her that he would recover Yuu for her and send the two of them home if she helped him revive the Darkness Loader.Later, when Sparrowmon finds Xros Heart in the Dust Zone, he explains that Yuu is being held hostage by DarkKnightmon himself.

Phineas and Ferb A-Xros The Digital World
Yuu, many years before hand gets a X Loader, and re-enters the Digital World and helps the evil DarkKnightmon and Bagramon, he helps Xros Heart, and returns to Japan, after that.

Trivia

 * The creator states that Yuu is based on Yuu Amano from Digimon Xros Wars.