Which Xbox game introduced the character Marcus Fenix?

Gears of War introduced the character Marcus Fenix.

Epic Games developed Gears of War as a third-person shooter for Xbox 360, where it launched in November 2006. Players control Marcus Fenix, a former soldier who returns to duty during humanity's war against the subterranean Locust Horde.

The game became known for its cover-based combat, heavy weapons, squad-focused campaign, and dark science-fiction setting. Its cooperative campaign and competitive multiplayer also helped establish the franchise as a major Xbox property.

Gears of War 2, released in 2008, continued Marcus's story, while later entries expanded the series' timeline and cast. The franchise was originally titled Gears of War, although later releases commonly shortened the branding to Gears.