Merlin does not like Gwyn since she helped bring down Camelot, but I do not believe it is a all-radiating hatred or anything like that. It is mostly a muted sort of tight-lipped disappointment.
They can be acquainted through boarding schools, he was attending one with Sigyn. So the three of them can know one another? We can start a boarding-school club. Ha.
Hades and Merlin may get along in the weirdest sort of quirky way.
Hermes can post pedobear pictures at Merlin, please and thank you.