Trill is a baby NetNavi and then later a young child one, he is an anime-exlusive character that comes from the world of Beyondard a parralel world ruined by a Dimensional Area accident caused by that world's Mr. Wily.
He is known by the Zoanoroids as The Synchronizer for he contains the power of the Beast Factor, frequently throughout Beast he fuses with Megaman sending him into an uncontrollable Beast out state. Because of this power, Zoanoroids constantly attempt to steal him. He escapes to the regular world but they chase after him there. He meets up with MegaMan.EXE and preforms Beast Out with him to save him from the Cybeasts. In other Beast Out attempts, he goes out of control. It's only later thanks to Iris.EXE that the Beast Out can be controlled. After intense battles in the regular world, the fight is taken to beyondard where they attempt to find trill and keep him safe. After defeating the beyondard Lord wily, Lan and MegaMan bid farewell to Trill when they leave Beyondard.
Trill's last two apperances were in the final of episode of Beast when he said goodbye to MegaMan.EXE A presumed fake of him brought on by Cache's data in the last episode of Beast+ appears and saves Lan and MegaMan from their fate before disappearing once more.
Trill's name is also a musical term which means going from one pitch to another rapidly.