Regulus Black (they got off on the wrong foot when Regulus called him by the wrong name)
The Bump aka The Hairball (niece or nephew)
Remus Lupin (brother-in-law)
Severus Snape (twin brother)
Severus and Serenus' birth mother died under suspicious circumstances when they were 13; Severus believes his father is responsible for her death. Their father is still alive, but he doesn't know about Serenus. Their mother's name was Megaera, and they had two aunts on their mother's side, Alecto (who raised Serenus) and Tisiphone (fate unknown). Serenus grew up in a small wizarding village in rural Italy, and he didn't know until his aunt was on her deathbed that she wasn't really his mother and that he had a twin brother. She died a few months before he arrived in London in October, 2004.
Nymphadora Tonks (he's the godfather of her twins)
Loathes With the Searing Heat of a Thousand Suns:
Remus Lupin (once, here)
Draco Malfoy (once: threesome with Harry and Draco here)
Harry Potter (once: threesome with Harry and Draco )
Severus Snape (once, here)
Nymphadora Tonks (twice: here and here)
Charlie Weasley (once, here)
Percy Weasley (once, here; he got a tattoo -- now gone due to Hyde -- thanks to Percy)
Blaise Zabini (past relationship; first time here)
He had other lovers prior to his arrival in London, but they were short-term flings. His first serious relationship was with Blaise.