I've been contemplating if he'd bring people from the holy faction back to his world. But I doubt it unless he creates a way to dominate them like the Blood Crystal to the Vampires. The Saintess has clearly been at this for a very long time, decades or even centuries. While she may be forming an attachment to the Holy Knight and maybe even Romantic feelings. It'll probably pale in comparison to her own morals and religious beliefs. She'll likely see the MC himself as the source of all evil who's using their world as his own personal playground. At the end of the conflict he'll probably Fake the Holy Knight's death in order to extract him.