- "This is what assault looks like! One more word out of any of you and I'll have you banished! I mean you, too Mercer! Everyone, back the fuck off."
- —Sebastian to Mercer along with Michonne and the group.[src]
Sebastian is shown to be a spoiled, lazy, self-entitled and deeply pretentious young man. He uses his mother's position as leader of the Commonwealth as leverage to get away with his deplorable behavior. Sebastian thinks that people should be grateful to him, as he believes that he is making the Commonwealth safe. Sebastian is also implied to be unfaithful, as several other "girlfriends" are mentioned aside from Kayla, and he is shown to flirt with Yumiko. Pamela believes that Sebastian's behavior is the result of his father dying.
Nothing is known about Sebastian's life before or as the outbreak began. He may have lived in or near Ohio.
Sebastian is first seen with Kayla outside the Commonwealth. Their encounter is interrupted by Mercer, who steps in to save them from a roamer. He later complains to his mother about his lack of privacy.
Sebastian later meets with Eugene's group and tries to flirt with Yumiko. Angered when the latter rejects his advances, Sebastian starts manhandling Yumiko, shoves Magna to the ground for trying to intervene, and causing to Princess try and attack him. Outraged by the altercation, Sebastian demands Mercer to banish them all from the Commonwealth, but Mercer points out that Sebastian had provoked them first. Elodie then tries to help Sebastian up, only to be slapped by him, much to Michonne's anger.
This list shows the victims Sebastian has killed:
- Possibly numerous counts of zombies
Pamela and her son have a somewhat complicated relationship. Though Pamela clearly loves her son and lets him get away with his obnoxious behavior most of the time, she is likewise disappointed in him and encourages him to be more responsible, as she believes he is destined for greater things. When she took part in helping with cleaning up the streets, he is shown to look down at her for helping the people out.
Sebastian has a poor relationship with his body guard. Sebastian is shown to be verbally abusive towards Mercer for ruining his date, even though Mercer saved him from being killed by walkers in the process. Mercer hates Sebastian strongly, but holds his tongue when around him. Mercer also believes there is no reason to kill Sebastian as he will likely get himself killed. It's possible that Sebastian looks at Mercer as well as the other solders as expendable. Sebastian is also shown to be intimidated by Mercer when the officer becomes visibly angry with him.
Despite being in a relationship with her, Sebastian isn't particularly faithful to Kayla, as he tries to flirt with Yumiko and is implied to have several more "girlfriends".
Sebastian initially finds Yumiko attractive and tries to flirt with her but is quickly outraged when the latter rejects his advances. The two have a poor relationship from that point on.
Magna and Sebastian have a poor relationship, with Sebastian shoving her to the ground when she tries to get him to let go of Yumiko.
Juanita and Sebastian have a poor relationship, with Juanita threatening him with a spear after the latter assaults Magna.
Elodie and Sebastian have a poor relationship, with Sebastian slapping her just to prove a point. Despite this, Elodie stops her mother from drawing her sword on Sebastian, reasoning that they "have to know their place". It's entirely possible that Sebastian looks down on Elodie as expendable, similar to Mercer.
Michonne and Sebastian have a poor relationship due to Sebastian assaulting her daughter.