Her name is Van Gogh, unless they plan on doing some sort of shenanigans like another ancient God masquerading as another person again.
You can read the spoiler in the wiki or at her page in the FGO wiki for her information if you want, unless you can read japanese from the game itself or want to wait maybe a year or 2 until the event comes to the English Server. Of course it's just an abridge version and I think it's best to read the full translated text for the event and its story which isn't too bad. A little strange and depressing but that's just the usual FGO stuff. It should be on either reddit or youtube, I believe.
I'm Gogh.Master-sama has entrusted me with an infiltration mission.It is a mission only Gogh can perform.people than I expected...I am very happy to be of use to Master-sama in any way I can.Ah- there are morepanting