User blog comment:Skeith9/Heroic Spirits Speculation/@comment-50.159.54.248-20140711033126/@comment-5739199-20140711205605

they can be both Heroic Spirits and Divine Spirits if they died before becoming gods, like Heracles, who only became a god after he died, so he was able to be summoned as a Servant

someone like Sun Wukong can't be summoned as a Servant because he is immortal, thus never died, and then he was granted Buddhahood (basically turning him into a Divine Spirit).

If the Boddhisattva Guan Yi died before becoming a Deva, then depending on her deeds as a human she could be a Servant.