Friends/Former Classmates:
I expect even a man like Marvolo would have some friends, mostly purebloods who respect or even admire him (or are terrified and simply want to be his pal to ensure they don't become targers of his anger maybe?) or at least share his views about Muggles and none Purebloods. He was a Slytherin and attended Hogwarts during years 1881-1884, not coming back after finishing his 3rd year.

Enemines:
Likely any Muggleborns, some if not all halfbloods of any class who don't give him respect. He'll be more civil if treated at least as an equal, but he'll not take kindly to being looked down upon especially by non-purebloods.

Romance: Marvolo is happily married to his cousin Morgana and unlikely to be lured away from her. Ladies are welcome to try, assuming any would find him worth the effort. He is a descendant of Salazar Slytherin, that may be enough for some I guess

Feel free to respond below or DM my main account Walter Staghart