Tazuki is Sora's closest friend and basically the responsible one of their group. He's smart and constantly helping Sora with schoolwork, which tracks given how much Sora needs it.
When Mii first showed up as this impossibly cute tiny mummy, Tazuki had plans to mess with him—but that lasted about five seconds. Mii's cuteness is apparently weaponized, and Tazuki caved hard. Now he's just protective of the little guy like everyone else.
The oni kid Conny? That's a whole thing. Tazuki found him on the way home from work and, if we're being honest, his first instinct was to toss him out because the idea of caring for a kid freaked him out. He got over that pretty quickly though, and he's the one who actually named him Conny.
Here's the thing about Tazuki though—he's chill most of the time, but if you threaten anyone he cares about, he becomes genuinely terrifying. It's this switch that flips, and people who see it tend to take it seriously.
Content compiled by AnimeList.moe from publicly available sources.