Jacob, the Father. A lot is unknown about him. He has severe anger issues and is extremely abusive. He's intensely LGBTQ+phobic and even racist. He left one day when tristan was 15. He was single-handedly the reason that tristan was such an asshole.
Charlotte, the Mother, is a very nice woman. She does all that she can for her kids, and stayed with Jacob because he was good to tristan and she couldn't rip that relationship away. Even when he abused her and left emotional scars that may very well never leave, she stayed with him for her kids. When Jacob left, she made it clear that she would be there for the kids.
Aleiah is Tristan's younger sister, she's still very childlike and happy. She was the main reason no one could ever lay their hands on tristan, because she would cling to tristan like her life depended on it and begged for the person Tristan made mad to leave him alone. She's very attached to Tristan. She's basically the only person who didn't experience Tristan's personality change from taking after his mother to taking after his father.
And Tristan. Tristan is 15 and actually used to be a very nice person. Then he turned 12 and started to take after his father. When his father left, he realized what an asshole he was. he'd been hurt by his father so many times, but he didn't realize what it was until he left. He's now trying to fix his messes and become a better person.