I was thinking the same dark forces that are sending the winged girls would put a hit out on him. Double their chances of snuffing him out before he reaches his full potential, or even realizes his potential. It is not an absolute necessity for me to have the contract but she is already close to him. She doesn't really need to know the reason, in fact she normally wouldn't.
Iseldra could also join later, I don't necessarily see all the girls converging at the same moment. I don't know that she would be banging on the door in the middle of the night, but I'm flexible especially on a starting scene.
I could see them having to defend the tower against the winged invaders until the wizard arrives, perhaps he knows something of Martin's destiny.
Let me know if you want me to write another post for my girls. I could see her on a roof top with Martin in her sights as the winged girls attack.