What a horrid use of a character. If she was just a teasing senpai that showed up occasionally it would've been fine. Instead, she had an express interest in the male lead and almost exclusively acted to either further her relationship with him or hinder his relationship with the female lead.
It isn't even used in a way to add real drama, it's just straight up annoying. The second the female lead wanders off, senpai is macking on him. The second the female lead starts to make progress, senpai is teasing and distracting.
She even keeps being a pest after explicitly being turned down, and the damnable unassertive male lead isn't willing to put his foot down and just tell her to buzz off!
It's incredibly bizarre, it's not a thing where this series has a big ensemble cast and this is just one of the characters, there's literally just the two leads and senpai as regular characters. Why does she exist?!
This series wouldn't be a banger even if she were removed -- it's somewhat generic and the art gets sloppy repeatedly -- but I'd at least have given it a seven, still.
Want something similar but better? Read Tomo-chan is a Girl!