An invisible and amoral cupid could do a number of wicked things without consequence. I wouldn't be shocked if it comes to light Sae orchestrated all of Hasumi's misery personally, using cupids' innate charm ability to turn classmates against her and likely personally pulling things like
https://dynasty-scans.com/chapters/love_bullet_ch13_1#7
when no one was looking. Heck, you can see the teacher's eyes go funny when she leans in to talk about pizza, then the next page he forgets Hasumi exists. Considering Koharu was able to Jedi mind trick people with it on her first day after brief off-screen instruction, one must wonder how much a mortal mind can be made to bend by a cupid who focuses on that sort of thing.
https://dynasty-scans.com/chapters/love_bullet_ch04#13
I don’t think it was a love plot, simply because of just how isolated she had Hasumi. Maybe it’s some elaborate revenge plot for reasons we don’t yet know about? Or maybe cupids can become overwhelmed by jealousy and turn bad, and something about Hasumi drew Sae’s attention in particular? Heck, maybe she’s just gotten bored/gone a little mad, and started messing with people’s lives for her own amusement.
Either way, I’m excited that we might be seeing a proper villain for this series. I’m pretty sure she has some sort of mind control, considering just how easily that teacher went along with whatever she said, and she’s experienced enough that she was able to work on multiple couples at once.
Extras have more than once shown cupids with green eyes, hinting at some manner of mystical jealousy going on in the verse.
https://dynasty-scans.com/chapters/love_bullet_twitter_extras_english_license_announcement#16
Green eyed Kanna in particular raises questions.
https://dynasty-scans.com/chapters/love_bullet_twitter_extras_english_license_announcement#19
Ena's timeloop in particular is a real mindscrew to me though. Since one might think being stuck in one would mean you are effectively isolated from the rest of the world until it resolves. But that is evidently not the case; her and Chiyo are familiar with each other despite this, and she herself notes she has more flight experience than Kanna. She's been at this for a while, and other cupids occasionally wander into the closed loop and back out again, apparently? But if she's stuck at that point until she resolves groundhog day, then wouldn't everyone else never progress past her until she does? But if they did somehow go on separate from the loop, how the heck would anyone manage to get back into it? Cupid Time Travel powers would be a hell of a thing to throw in... but even as I type this I can't help but note that Koharu and Kanna fell asleep on the way to where they are now. If Chiyo had to pull arcane shenanigans to get back into Ena's loop, that'd be the time to do it.
Because without some manner of time magic even beyond the existence of the loop itself, I don't see how the timeline can make sense without Ena just being trapped in some kind of simulation/hallucination instead.
Cupids are reborn on their birthdays, some amount of time after their death. Ena shows us that amount of time can be ~0. The bio sheets show us that Koharu's birth month is March, while Ena's is July, with nothing indicating to me that Koharu has been at the Cupiding grind for months by now.