There's something wrong with her for sure.
But what I'm really liking about this series is that it has real progression. It's set up like Tejina-sempai, but instead of bullying moments replacing the repeated ecchi gags that are reset each time, there's change in both characters.
Nagatoro gets more moments of genuine embarrassment when her prods backfire, or when they're completely alone, there's some level of kindness and fondness. She comes off as just lonely during the video game arc, even though her family seems pretty big.
Senpai is getting more daring, is starting to gain some level of confidence, and even before the manzai segment, was learning to push back on her attacks.
Nagatoro's dead stare though...she's possessive for sure. She's the only one allowed to tease her 'boyfriend.' The pigtails blonde friend seemed happy enough for her, but the redhead immediately caught what's going on.
Her trigger getting flipped when he starts to cry still strikes me as Nagatoro is too awkward to know how to handle when she's gone too far. She just plows forward instead. There was a moment of hesitation and embarrassment that started with half of a spoken apology, then a mental deadlock indecision moment (pretty recognizable from Nanashi's art) before she decided to double down.