I feel that Glossaryck is the best written adult character in the show. He is never overbearing in his importance and he is never showy with his power. However, just with his subtle ease of magic usage compared to Star, we're able to easily gather enough of an idea of his true power that we aren't surprised by the later revelation that he's the strongest magic user in the known everything.
But, I think the best part is how believable he is as an adult. Compared to even all the human characters, Glossaryck is the most believable adult in the series. Even if he has a jester-like free spirit to him, Star and the audience feel safe whenever he's on-screen.
You know that, even if bad things happen, Glossaryck has the willingness to bail Star and Marco out of trouble and he never lets Star handle things that he knows that she can't handle.
So it's no surprise that Star became weaker after losing Glossaryck and we as an audience are able to feel the impact of his absence.
Glossaryck is the best adult character.