Rivalry

Rivalries are a recurring theme in the Mega Man franchise. In each series, the hero often has rival in which they compete with to determine who is the superior being, whether they are willing to or not. Rivals can range from friendly to hostile, but in certain situations are set aside to deal with a mutual threat.

Original Mega Man series
Mega Man and Proto Man were originally rivals.

Mega Man and Bass

Mega Man X series
Mega Man X and Zero.

X and Vile.

Zero and Colonel.

Mega Man Zero series
Zero and Craft.

Zero and Omega.

Mega Man Battle Network series
Lan Hikari and Dex Oyama.

Lan and Eugene Chaud.

MegaMan.EXE and ProtoMan.EXE.

MegaMan and Bass.EXE.

Mega Man Star Force series
Geo Stelar and Omega-Xis and Solo and Laplace.

Mega Man cartoon series
Dr. Light (Ruby-Spears) and Dr. Wily (Ruby-Spears).

Mega Man (Ruby-Spears character) and Proto Man (Ruby-Spears).

MegaMan NT Warrior series
Lan Hikari (anime) and Dex Ogreon (anime)

Lan and Chaud Blaze (anime).