1. You can read here about the void https://itch.io/t/4863660/lags-i-cant-go-to-the-void
2. It will banish the ghost in the map but it won't capture her, just banish.
3. IIRC, you can find it in the school first floor all the way to the left.
Bonus: To help to defeat her, when you are at the verge of losing (I think its 45 pts each round, not sure with the new update) just go do your thing at her by going to the other side of the net but take note it will reset your current points and not round points of the match.