This is a pretty weird reading of the character and the situation around her. You bring up her romantic feelings for Guts but fail to bring up the fact he shuts her down hard without any question to his feelings on the subject. She might have some lingering feelings for him buts Guts isn't and never was about that. She's also a vital member of the party and being a skilled magic user she's often more important and essential than Guts himself.
Not even sure how she doesn't fit in tonally when there are several characters that are far more humor focused than her like Magnifico and Isidro and she isn't even the only younger character in the group. If anything Guts is the one who doesn't fit tonally though that's kind of the point considering who he is and what he has done contrasted to the rest of the group.
Why weird reading? Guts doesn't fall for her, else I would've mentioned that because that'd be a larger issue. But it IS accurate that she's a cute lil' wizard girl falling in love with Guts and there's no way that anyone can tell me that there isn't a tonal difference between the Golden Age arc and the rest. I mean Puck alone does amusing reaction faces all the time and once Schierke joins, with her own funny elf no less, that only amplifies.
But you say it yourself, Guts eventually contrasts with the rest of the group he's working with - some group you could find in any generic adventure story,
braindead