Sebastian is a rebellious loner who lives in his parents’ basement. He is Maru’s older half-brother, and feels like his sister gets all the attention and adoration, while he is left to rot in the dark. He tends to get deeply absorbed in computer games, comic books, and sci-fi novels, and will sometimes spend great lengths of time pursuing these hobbies alone in his room. He can be a bit unfriendly to people he doesn’t know. Could a charming new farmer cultivate the wasteland of his heart? Who knows?

Source - Stardew Valley Dev Update #12

Sebastian lives with his mother, Robin, along with his step-dad Demetrius and half-sister Maru. Robin sometimes calls him Sebby. He feels that Maru is treated better than he is, and during Winter will talk about building a snow-goon that Demetrius demands he removes; he will then angrily question what his step-father's problem is.

Sebastian is good friends with Sam; the two can be found hanging out at Sam's house or at The Stardrop Saloon playing pool. They also stand together at festivals, along with Abigail. It is hinted he may have a crush on Abigail, after Sam tells the player Sebastian really wanted Abigail to be their drummer "for some reason". Furthermore, if married to Maru, she may sometimes say she "wishes her brother would just ask Abigail out".

During the Flower Dance, if not dancing with the player, Sebastian dances with Abigail.

Source - Stardew Valley Wiki