Let me guess, she's actually his disciple from before passing away, also reincarnated as a maid!
If it's a reincarnator the only real options are the Sister the MC saw in a dream early on and the Master he studied under. But they were only like 1 page each so it doesn't seem like they'd be important enough at this point to be some big reveal reincarnator. The girl probably has no connection to his past life, in all likelihood she's probably some childhood friend in this life, it seems the MC only remembered her past life right before the story started. Either that or she's some kind of weirdo lesbian pretending to be incompetent to force the MC to constantly personally tutor her and thought the MC was finally catching on.
My bet is that she is just someone she knows in this life and is also interested in Yuri shenanigans, and will likely do something to stake her claim and piss off the other girl or else reinforce the misunderstanding.
"Annette! What are you doing?"
"Milady, it's not what it seems!"
"Yes milady, Senpai was merely going to discipline this poor maid for getting her wet~"