You can be talking about male. They can be a human or an animal. I don't think I have ever met a girl with the name Sebastian though so, we are going to use the masculine words (words ending in a are feminine and words ending in o are masculine). This person/animal is going to be a male. I am going to pick an anime character for this.
Sebastian is very handsome. Sebastian es muy guapo.
From his black flowing hair and whine red eyes.