Okay, after looking into a whole bunch of twitter posts (and i mean Likes/Replies from back to 2021)...
I have finally found the hair in question. It's from an avatar called Tycho - here's a pic:

Sadly the Avatar seems to be not available anymore: https://booth.pm/en/items/3123433
But we seem to have it on here, thanks to @eiron : https://forum.ripper.store/topic/489/looking-for-ruginea-tycho/5?_=1712801171985

I don't know if there's a newer version, but who cares if you're only interested in the hair.
Anyways, I'm gonna get some well deserved rest now - Had enough Twitter for half a year now xD

@Sheiramoth Checking both models again, you're actually right. It must be Kikyo.

@FuFox a friend sent it to me. He said he got it from Pixiv but doesn't remember the user anymore.

So after checking around for a bit, reverse searching myself into a rabbit hole, I might be on a lead here.

• The avatar is most likely Manuka, only the big toes on her feet do look different (maybe due to perspective).
• The cat ears are most likely from another avatar (I have no clue which one, looks a little like Maya's but not exactly).
• The hair could be this one here minus the ponytail: https://booth.pm/en/items/4752745

But than again: The ahoge on top of the head is missing, the ponytail is either not visible or removed and the front side of the hair seems off (could be blendshape work, need to try it out myself).

I did find an upload for the hair on these forums - special thanks to @starcrafter
https://forum.ripper.store/topic/8775/lf-this-avi-s-and-package-5-9/33?_=1712783352203

As I am not 100% sure yet that this is the hair I am looking for, I'd like to leave this topic open. Maybe there's someone who has a better lead.

Hi everyone, can you help me find the hair and avi from this picture?
I do have the suspicion it's Manuka but I've never seen the hair on my booth tours before.

+bump

Does anyone have the rindo version for this, please?

+1 bump