Iroha's a first-year in class 1-C at Subu High, and she showed up pretty late in the story—volume 7.5. Hachiman's take on her is kind of funny: he pegs her as either "not-cute Komachi" or basically "Sagami but actually competent," depending on his mood.
The thing about Iroha is that she's got this whole carefully maintained cute-girl image she puts on for everyone around her. But with Hachiman specifically, she just drops the act. He's one of the few people who gets to see who she actually is underneath all that.
Content compiled by AnimeList.moe from publicly available sources.
