Excalibur

Excalibur is a sentient legendary sword said to predate the demon weapon race entirely, and by most accounts one of the single most powerful weapons in the world. His compatibility with virtually any soul should, in theory, make him the perfect partner for any meister.

In practice, it never works out that way: Excalibur is so insufferable to be around — condescending, short-tempered, and obsessed with reciting his self-authored "thousand rules" — that even meisters who can wield him eventually abandon him out of sheer exhaustion. Characters like Hero, who seek him out purely for power, quickly discover this the hard way.

That combination of extraordinary strength and total unbearability makes Excalibur one of the story's most comic characters, while still leaving him genuinely important to the plot.