Stannis was a dandy who enjoyed his wealthy life as heir to one of the wealthiest families of Selgaunt.
However, everything changed in 1345 DR when Stannis and his mother, the sorceress Velanna Malveen, fled from Selgaunt after the former was discovered having had dealings with pirates. Their ship was destroyed at sea and Velanna was killed, but not before summoning the aid of some sort of aquatic vampire, who rescued and turned Stannis into one of its kind.
Stannis went back to his family's estate with his new master. Here, however, Stannis's brother, Radu Malveen, despite his young age, easily slew the vampire master and Stannis decided to follow Radu's will.
Always a dandy, the years of isolation made Stannis exaggerate this characteristic. He was obsessed with revenge against the Uskevrens for them not helping the Malveens in 1345 DR. Stannis obeyed and feared Radu.