there was a Q&A with the author where she states that all humans have black hair, and that the usual "dyed hair" you see in manga wouldn't apply to this series--and that along with the overall darker tones was her way of differentiating humans from cupids.
So Sae, even seen in this flashback,
not having black hair like everyone else, tells me she's a standout of particular significance. She also is clearly leading the bullying against Hasumi, and everyone else simply follows along.
Plus, we see her with those "Charm Eyes", reflected in the sensei's when talking about the pizza party thing. The rest of the class also praises Sae for being so successful in convincing the teachers to go along with whatever she suggests.
That's pretty indicative of that Charm effect, as well - and as you said (since I just read through everything today), in previous chapters, the "student Hasumi ran after" wasn't named. So Sae's been forgotten.
The only weird thing that I don't know can be squared yet, is that she appears in that photograph with everyone else. Would her "disguise" be preserved even after the fact? Would she be recognized by her classmates as "Sae-chan" even if she otherwise no longer exists for them? I think we'd need one of them to look at the photo and pick her out to be certain, and that hasn't come up yet.
But I'm pretty confident that yes, Sae has been a disguised cupid. She also was the one who suggested the pizza party
early, which prompted the ahead-of-schedule hike up the mountain.
What if she also was responsible for the earthquake, and the resulting landslide? Peeling off from the group, so that she'd be "missing" when it happened, knowing that Hasumi is responsible and dogged enough to look after even her bully in an emergency? This whole thing could have been Sae working to get Hasumi killed.
She knew where to find her post-cupidization, after all.
To what end....I think there are a couple routes.
- Your guess is a reasonable one; plenty of people IRL believe in "OTP" type love stories, and someone sufficiently obsessive about it could turn to violence against would-be "transgressors". Though Sae going all the way to seemingly killing Hasumi, and tormenting her so much in life. That feels incredibly personal, so simply warning off Hasumi from interfeing would have taken a lot less. (the whole "hatred isn't the opposite of love, but its twisted equal" type thing - the amount of bullying Hasumi suffered at Sae's hands is far too intimate, in my mind, to be a mere erasure of a love rival for someone else.)
- I could see it being that Sae herself has feelings for Hasumi, but something got twisted, and it shows up in this perverse manner. Perhaps Sae herself has something going on mentally, that has skewed her way of dealing with people? She herself is a cupid, meaning she met the same conditions as the others, in death.
- The most off-the-wall, would be that Sae is someone from Hasumi's past even further back - someone older than Hasumi (since cupids don't age after death, apparently), who died and blames her death on Hasumi for whatever reason. And now she's come back to bully and torment Hasumi, trapping her in death as a cupid to "punish" her.
I actually kinda like this one, or at least some iteration thereof--after all, Sae's the same "age group" as Hasumi at the other classmates, but if there's a "cooldown" between death and cupid-birth like with Koharu, then Sae had to have died some time ago. Though then again--Ena herself seems to be caught in some kind of "loop" with her own death and aftermath; so there might be bigger things at play that we don't have context or mechanic-knowledge for.
edit--another idea: Sae had feelings for a younger Hasumi, died, and is now trying to trap Ena with her as cupids. Sure, they can't feel love--but she can now have Ena all to herself in the afterlife, and show her "extreme attention", even if it's not a positive loving kind.
It's compelling, though. I know I saw that it appears that the author has planned this whole story out--and that Ena's own arc will take multiple volumes (started in vol.2, but could go as far as vol.4, perhaps). So we'll be here for quite awhile it would seem, and that's leaving aside Koharu's own character narrative, much less Kannas or Chiyo's (who are probably going to be just as complex, I figure).
For an introductory work, this is very ambitious on her part, and I'm excited to see where this goes.