Onion Knight is meant to be weak, IMO he's got the most balanced skillset of all characters.
Four brave attacks, two aerial, two ground, one each of which is magical or physical.
Four HP attacks with the same pattern
Four brave-to-brave attacks, that is, his initial brave attacks can chain into another
Four brave-to-HP attacks, easy to figure out what that means
So basically, he's extremely versatile, his attacks chain into each other, he's got both magic and swordplay in air or ground. He's got a lot of awesome combo potential.