Adui
So far it's interesting but already suffers from idiot MC syndrome that seems to be a requirement in 97% of stories. Like everyone says, he should absorb the cores and also learn the healing spell in case something unexpected happens and he gets hurt. Plus with the way he can level up low spells to have higher damage then higher tier spells, his need of a higher tier spell is not as urgent. But of course that would be the MC using common sense and not being an idiot so he can't for that route.