The faithful companion to Brad Evans, found as a puppy in the forst of Green Hell. Rassyu leads Brad to the relative safety of the town of T'Bok, and comes under the care of Merrill for the years while Brad is imprisoned. Once released, of course, Brad finds Rassyu as a full-grown dog still waiting in town.
Oddly, once Rassyu is reclaimed, he's not mentioned in the game again. It would have been a really convienient way to spot the doppleganger Brad during one of the game's pivotal sequences...
As with many things in Wild ARMs 2, Rassyu's name is a horrible mistranlation of the Romanji used. It's likely that the name is meant to be 'Rush', which is in keeping with Brad's special attacks which make use of him.