User blog comment:Hitsuji Mamoru/Servant Creation Room 2 : Guess of the Who/@comment-31726296-20170501023813/@comment-4802891-20170502030821

Alteregos and Mooncancers should be fine, but they are a lot more difficult to come up with. Mooncancer maybe not, because you could essentially take any Heroic Spirit and say it evolved into a free AI corrupting the Mooncell, but Alterego has to be an aspect of an existing Heroic Spirit/known character to be relevant (unless the game comes up with some random stuff in the future).

I wouldn't risk myself at writing an Alterego, that's for sure.