Who is Hanazawa Hanako?
Hikigaya asked for a girl's phone number in middle school, as mentioned in his flashback at 6:30 - 7:00 in episode 4. The number probably belongs to her, though he left her anonymous in his story in the anime (I haven't read the early volumes of the LN's). There's no Hanazawa Hanako in Volumes 7-10. For now, she remains in the back-burner of possible characters.