Fairy Tail

[Buffalobiian]

Also I'd advise not to underestimate Lucy, she's quite the fighter when she puts her mind to it

Which makes me sad how Fairy Tail chooses to not explain character powerups. Summoning Mages had to increase their capacity if they wanted to keep more and more spirits in the human realm, for example. Without a system with which characters can grow by, I can hardly gauge how they've grown at all.