You need to play the first scene with that girl using your bear or cloy, and then, when you are able to select what do you want to do: continue, change monster, let her go, finish her.
You need to select 'finish her'. In that way you will start the scene between Rex and that girl. If you have enough energy you will have the option to use spells on the girl and/or Rex.
Also, you can level up your spells until each spell have a cost of zero energy.