Rush actually appears in the 1st Season, but not until the very end. His role in the 2nd Season however, is much bigger, thus I am sticking him here. Rush is a virus program, created by Netto's father Yuuichiro. He originally made him so that he could help the Navis track down Rockman during episode 24. Afterwards, he became sort of a "pet" for Roll-chan.

In episode 41, the Neko Virus scenario, Rush gains the ability to project himself into the real world via hologram. Afterwards, he keeps this ability, and reappears quite often in both the net and the real world. This continues even in the new season of Axess. ^_^;;

In Stream, Meijin forcibly gives Rush a new trick. Infusing the dog's powers to trasmit himself into the real world, Rush gains the new ability to turn into a special "Synchro Chip" just for Meiru, who has troubles using the power of Cross Fusion. Relunctantly, Rush will transform into this chip for her, and together they fight against evil. Rush doesn't really like to fight though, so sometimes it takes a little coaxing for him to agree to it. ^_^;

On the plus side, with the addition of Rush, Meiru also gains some of his more unique powers. Most notably is the "Rush Hole!" attack, which allows her to suck her enemy down into an endless void.

Source: Rockman.exe Online