[Favorite trope] Character has a badass well known alias, but their real name is extremely mundane

The UnderTaker; One of the most brutal and terrifying wrestlers in WWE, pray you dont see a coffin near the ring. Real name; Mark Sol Badguy; Decorated war hero, ruthless bounty hunter, killer of godlike beings, and quite literally THE guilty gear. Real name; Fred Guile; Badass soldier, countless medals of honor, goes toe to toe with demigods, and can shoot literal sonic booms from his arms. Real name; William