actually, she says that he's more mature than her, but that's linda true due to fact that he has a mind of 30yo. Natsumi and Hiragi parents also thinks that Seiji is a mature person. However, we didn't see Sana saying this. About Seiji's classmates, he only interacts with Fujimoto, but they don't seem that close, maybe it's because Seiji knows they won't Interact again in the Future.