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.