D.Gray-man

Cursed with a left eye that lets him see the souls trapped inside Akuma — mechanical weapons built from the dead by a villain called the Millennium Earl — Allen Walker can never ignore what he's fighting, because he can see exactly how much those souls are suffering. His left arm transforms into an Anti-Akuma weapon powered by a mysterious divine substance called Innocence, and after years of brutal training under a general who was, by most accounts, a terrible mentor, Allen joins the Black Order — a secret organization of exorcists fighting to stop the Earl from ending the world. The setting is a gothic, fog-drenched alternate 19th-century Europe, and the whole show leans hard into that atmosphere. If you liked Fullmetal Alchemist or Soul Eater, the bones are familiar — young fighters, a dark organization, a power system with real rules — but D.Gray-man has a heavier, more melancholic tone than either. Allen is carrying guilt that shapes every fight he takes, and the show doesn't let you forget it. At 103 episodes from 2006, it's a commitment, but the early arcs establish the world and the stakes without overstaying their welcome. It's not flashy or loud. It's the kind of show that gets under your skin slowly.