That's Japan for you. Speaking informally to people you're not very VERY close with is considered extremely rude, which is why you'll notice very often the girls are much more relaxed with their speech amongst each other, meanwhile Maki is much more stiff, because before Umi he didn't have any friends, so formal speech is his default (very common trope in manga where MC is a "loser" archetype)I’ve been reading this manga for awhile now and something has been bugging me for awhile now, these kids don’t talk like teenagers they talk like a business email. They always sound so formal, even Umi and Maki sound more like business partners instead of a couple. Idk but I just find that so weird.