She's Mercumenes, the majin servant/vessel-to-be of Lucifira.
https://mangadex.org/chapter/3216ffc5-dd56-4c69-948f-fd3573fb6e94/3
There's a short-ish fight with a different majin coming up (for once, Haruto get 'slightly serious')
Then due to Tia's made up story to cover up some truth (like how Haruto crafted the relic they brought back) they brought Mer to principal as proof.
Seeing Mer shivering in fear, principal asked to look after her for a bit which Tia figured that's the kind of person principal is and agreed.
Well, principal recognize who she is and Mer point out that principal is likely not human herself.
Principal concluded current Mer is harmless enough it'd be against her principle to kill her.
So she'll just delete her memories.