Liam is Ted's brother, and senth is teddy's son.
infodump time-
Liam and Ted are opposites; Liam is very aloof and apathetic towards most people and nearly all his relationships are transactional. whereas Ted actually cares about people, like a normal person, and is outwordly very friendly and extroverted and kind.
which is why he is nicknamed and goes by teddy ::]
(Liam takes it the extra mile to piss him off though, and calls him "T". you don't like how long your name is, don't worry I got it. *uses a singular letter*)
Now; senth has ADHD and takes a special interest in robotics for a while before it moves onto chemistry- but still goes to college with the intent of Mechanochemistry being his field ::]
that is before he died due to demigod fuckery.
Levi Tell; one of AAC's demigods, is a ghost who needs a physical anchor to keep him in the mortal plane. during a time when Liam was heavily deteriorated he just latched onto the closest person, that was unfortunately senth.
since it was unconsentual and unprepared, it shattered his body and physical makeup entirely.. and he became a soul.
while it anchored levi enough to find justin- it was really unfortunate for senth as he "disappeared without a trace"
(he's just sort of a soul with kayko that hangs around levi now since he doesn't have a choice. but like. he goes on cool adventures and sees other dimensions so that's fun at least)
Teddy suspects Liam killed him but Liam insists he didn't. and he isn't lying (he just killed different people is all- lmao)
this mystery is never solved and just sits with the three (liam teddy and Anne [ted's wife]) until the apocalypse.