Wow so we have some one with a crush on Yami. Lol this will get interesting. Yamis magic seems a bit low copared to others that is he doesnt have a wmd type attack likeothers. I suppose lnowledge of ki , physical strength and swordmanship makes up for the lack of such.
we don't know that Yami is weaker then the other. Yani's playing around hasn't been serious in this fight from the first panel of the first page he was introduced. The fact that he's a captain and the fact the main lightning villain said he's equivalent possibly to the Emperor King should not be taken lightly for all we know he's holding back.
When the pscycho mantis captain asked him if they were tough he said no there like little kittens so we know for a fact he doesn't take them serious and he only took this damage as his magic carpet suggested to save his comrades otherwise who knows what the hell he'd have done.
Throughout this fight Yami never has gotten a serious personality he's been screwing around and having fun even when it got three on one, Yami had to almost hold back so Asta, Gauche and his magic carpet didn't get killed I'm sure few have seen his full power in serious mode.
the fact that charlotte loves Yami is amazing this will definitely lead to some interesting and hilarious twists in the story and hopefully I hope they do marry. I mean if charlotte and Yami married and assuming the resulting kid had a combination of both powers he'd be a hell of a fighter using vines and darkness he'd be a real force.
Maybe asta will be the catalyst to get them married