Twelve/Bill
You don't know how happy I am to see you like this.
Right, so. The main thing to remember is Mels is River Song. Just, a little different. Alright, a lot of different. And she wouldn't know who I am. So for now, call me the Professor.
The TARDIS could keep her in here indefinitely, or at least until she hopefully changes back, but we run the risk of Mels getting violent.