
It's implied in the Japanese script and more so in the manga adaptation that Serges is Dr Wily revived in a Reploid body, though not outright confirmed.
Sigma himself is revived by the X-Hunters, starting his Joker Immunity.
The plot is to get back Zero's parts in order to revive him, and if you fail the X-Hunters revive him to serve as a boss. Avenging the Villain: The X-Hunters want to hunt down X in order to get back at him for defeating Sigma. The thing is that they respawn after you die, so you can fail as much as you want without a Game Over, and even fill up your Sub-Tanks for good measure. Anti-Frustration Features: If you're trying to get the Shoryuken in the third X-Hunter Stage, the path to it has an extra life, as well as a few health pellets. However, X will also discover a dark secret about the nature of Sigma, and that his influence may be more difficult to neutralize than it seems. Thus begins a war to get a chance at reconstructing X's dear friend. They also claim to hold the three pieces of Zero's body, and challenge X to duels over his control chip. Shortly afterward, however, the X-Hunters - a trio of Mavericks named Serges, Violen, and Agile - reveal themselves, unleashing a brand new group of powerful Mavericks. Under X's orders, the Maverick Hunters travel to the area to destroy them. X spearheads a crusade to root them out, discovering a large group of Mavericks in an abandoned factory. Sigma has been destroyed, but the remnants of his rebellion are slowly rebuilding their forces.
Zero's body is missing, and all they have is his control chip. In the year 21XX, set six months after the events of the first game, X has taken over as the leader of Maverick Hunters following Zero's death. Mega Man X2 is the second entry in the Mega Man X series, released on the Super Nintendo Entertainment System on Decemin Japan, January 1995 in North America, and October 1995 in Europe.