Honestly, I think Himawari has some traits that tend to give her away.
For example: she's not exactly the best at her job:
Or jobs considering she's been a Scarf Salesperson at the Mall, a Waitress at some French restaurant, and a Melon Salesperson at the New Years Festival.
Sensei has seen through her as Sana as well:
She tends to slip up.
Some other tells/traits that seem to belong to her:
- Doesn't actually seem to understand things, like the Resets, or what's going on.
- Has a desire for a designated cafeteria and has coworkers. Also eats Ramen.
- Has a connection to Sunflowers, Sensei, and Ayane.
- Has a fondness for Spanish related things. (Like Ayane.)
- Has a reason (to or) not to reveal certain things, which is implied to affect her future.
- Has blue eyes. (And probably likes blue considering it seems to be her color of choice)
- Is a Narrator.
- Isn't interested in a sexual relationship with Sensei.
- Is very positive and hopeful.
- Is working with and seemingly specifically protecting Ayane.
- Etc
There's probably other things like wanting to see things restored, not being the best at poetry, being jealous of Yumi's boobs, and being scared of Sekai, but those aren't entirely proven to be her, even if they seem likely to be her.
Overall: I think Sel would give at least a notable hint at her being there, instead of another character. It'd be up to the player to decipher it, though.
I'm still curious what was up with the whole offering Sensei some Goat urine to drink, thing, though:
Maybe she was trying to cleanse Sensei from something without being obvious about it?
There's also the whole Ami clone thing, but I don't think that's something we'll understand until we're supposed to.
