[yokai]

But is he the mum or the dad of Mitsuki? I just honestly thought he went on for too long, and didn't have much purpose until his revival and helping Sasuke to talk to the hokages etc etc

No one knows which parent he is, not even Mitsuki who is his son. It really doesn't matter as we know that Mitsuki is artificially created by Orochimaru. But as Orochimaru states and shows, he loves Mitsuki very much despite not being the birth father or mother.