Mirei's cute when she's serious. And any other time, really.
I like that she has her own convictions, especially in a climate like that.
On her side, it's not just that she likes to teach others (or maybe specifically him), but also that teaching is a great way to improve yourself. It helps you find places where you're weaker at, since you have to be ready to teach anything.
Great ending card!
I haven't really read series where the side characters outshine the main female lead in awhile,
I find that's reasonably common, though it used to be way more common back when harems started their popularity. Main girl was usually very safe and plain, while the other girls had actually interesting quirks. Nowadays more FMCs are more fleshed out with less plain stuff.