Sonic as Guy from Gaogaigar, and Tails as Mamoru from the same show.

Guy is a human that was saved from a near-death experience when an alien robot lion named Galion saved him from space, and brought technology that was capable of rebuilding Guy into the worlds most advanced cyborg.

Mamoru has a deep relationship with Galion, as they both come from the same planet, and their quest is to save humanity. Mamoru was not aware of this from a young baby till his elementary school days, when he discovered his amazing powers.

Together, they fight evil robot creatures named Zonders from destroying humanity. Galion allows Guy to fuse with him to make a mech named Gaigar, followed by a Final Fusion with other earth made mechs to make Gaogaigar, the King of Braves! With this amazing robot he can defeat the evil Zonders, while Mamoru has the ability to purify infected humans. He literally erases 'Evil' itself from someone's heart.

Guy and Mamoru have a very close relationship from the very beginning of the series. Shortly after they meet, Mamoru knows he will be around guy quite a lot, so he begins to call him 'Guy-nichan' referring that he sees him has a big brother. Guy is honored by this title, and he goes out of his way to make sure that Mamoru is never scared or harmed.