Keep in mind, I couldn't say the story is horrible especially now that I've caught up, but I'll still stand by my words that the trope grinds my gears. Just not annoying enough to stop reading this.
The main character is the straight man/comedic relief to the other characters' insanity and he has no real purpose other than just being there and occasionally teaching. It's pretty obvious given how often he points out that he's the only 'sane' one in the group and how he tends to overreact to the insanity going on around him. The girl companion is, to put it bluntly, simple- in a naive/dim-witted way -and a lot of her actions are made to come off as cutesy. A good example would be the petty acts that she does reminiscent of a child, her extremely easy-to-please personality when it comes to food, etc. All of which is very common with her character archetype in this type of story.
The only reason I'd assume someone would argue against her being simple is because she's canonically a genius magic-wise, other than that though... I guess her horrible past would also suggest a deeper character, but if anything, it actually makes her character archetype more obvious and, really, only enforces what I said in my first comment about this being one of those stories. They give their female companion a dark/traumatic backstory, then have them act the way I mentioned above.