It’s not just one Rush, either. The robo-doggo takes on a wide variety of different forms as the situation calls for it.
The most familiar one is probably spring-mode Rush, aka Rush Coil, where he transforms into a giant springboard and propels Mega Man to greater heights. It’s a lifesaver in certain situations and absolutely necessary if you’re playing spelunker and want to get everything a stage has to offer.
But Rush’s trick repertoire doesn’t end there, and it’s arguable that as the mainline Mega Man games started to lose their luster, Rush himself started getting even better.
Over the course of his long career, he’s mastered the art of turning into the Rush Marine submarine, carrying Mega Man through space as Rush Space, flying through the air as jet-powered Rush, and doing all the things you’d expect from a robotic dog with near-endless capabilities.
Rush’s loyalty is a bit different from most dogs, though. It’s hard to say he had a choice in the matter of staying by Mega Man’s side, when his creator, Dr Light, programmed him to be (Mega) man’s best friend.
But he goes above and beyond to fulfill his duties nonetheless, like the good boy he undoubtedly would be — y’know, if he could make his own choices.