What's the main differences between each Meta Knight?
UH-OHHHHHHHHHH THE FLOODGATES HAVE OPENED, here are some brief summaries i typed up at lightning speed (thank u for asking lol!!)
game meta knight: the "canon" meta knight. when you think about "meta knight" this is the guy most people probably think of. he represents every canon kirby game (including spin-offs), official merchandise, promotional material, books (such as "it's kirby time", but not manga or novels) and things like kirby cafe and pupupu train. he's cool, brave, good-hearted, and a bit of a dork. that said, he's serious to a fault and a chronic perfectionist, so he can be quite wayward…but when trouble arises, he'll be the first to know and the first to act.
anime meta knight: you know him. you love him. he's the retired war hero from the kirby right back at ya anime. i personally consider the anime and games to be separate, so anything outside of what's actually seen in the anime isn't canon to him in my hcs. on the outside, he's one of the most mild-mannered versions of meta knight. but at his core, he's fiercely righteous, and it can drive him to extremes. with unrivaled patience and resilience, he strongly believes in placing the keys in the hands of the younger generation. (literally. and then the younger generation ate them.)
smash meta knight: the meta knight with the ridiculously deep voice from the super smash bros series. his storyline includes major kirby games (no spin-offs or sub-material) and the events of smash bros. unlike the "canon" kirby universe, various video game franchises co-exist together in smash meta knight's universe. since brawl, he primarily resides at the smash castle to partake in smash tournaments, but he's called to action whenever dreamland is threatened. although he can be brooding, impulsive, and pompous, he's a truly steadfast companion, and ultimately wants to see others reach their greatest potential. his fighting style is much more aggressive than other versions of meta knight.
novel meta knight: the meta knight from the kirby novels by mie takase. his universe consists of the storylines from the novels, some of which include alternate/expanded versions of events from game meta knight's universe. he's a notably more serious, calculated and even potentially cold depiction of meta knight, but he's also one of the most deeply devoted to his crew. he acknowledges he isn't perfect, and can get carried away with things. unlike other versions of meta knight who tend to have a bit of a narcissistic streak, novel meta knight doesn't care for the spotlight, and even detests it. but, he always aspires to be polite (which is sometimes no easy task, given his no-nonsense philosophy).
manga meta knight: these are actually a multitude of guys. there are many kirby mangas, and every meta knight is portrayed differently in them, so that's probably a post for another day lol. but, my personal favourite kirby manga is the mo~retsu pupupu hour series, so that's the one i base my headcanons around. this meta knight is bombastic, flamboyant, pretentious and strictly adheres to his personal code of "love and justice". his universe is basically a chaotic, wacky and lawless mash-up of game meta knight's. things don’t happen the same way as they do in canon, much favouring comedy over drama.