Issac Rossum was a child prodigy and Ph.D in Robotics, who was killed prior to Mega Man Century by one of his own Robot Masters. He was respected by Dr. Light and Dr. Wily, which was strange considering Wily's usual attitude. He was the basis for both of the doctors and Dr. Cossack's creation Century Man, looking almost exactly like Rossum.
Rossum died prior to the beginning of the game and it is Mega Man's job of figuring out who killed him and put it to justice.
Rossum has a huge rule in this game.
When ending up in an alternate future, Century Man found Rossum's household, with Issac still alive. Rossum tells Century Man that his Robot Masters are in hiding and needs them back. Century Man does so, with a modification from Rossum that he can use defensive- and evasive-based weapons from the Robot Masters, since Century Man is incapable of attack. Rossum ends up figuring out that the reprogramming lock Dr. Light made in the regular timeline can only be opened by Rossum himself, even though he died in that timeline. He opens the lock and gives Century Man the means to defeat Millenium Man, the Time Buster.
Although he doesn't appear in the game, his parents are known for building Empty Man, and they found blueprints for Thermo Man, made by Rossum before he died, as a means to balance out Glacier Man and Hot Woman.