Spoilers ahead for 13 Reasons Why Season 3. All the kids on 13 Reasons Why have been through a lot, but few have had it as bad as Justin Foley. And even though he's now living with the Jensens and is in a relatively better mental space, his past isn't entirely behind him. When Seth starts following Justin in 13 Reasons Why Season 3, it threatens to undo any progress he's made.
Seth (or Meth Seth, as Justin calls him) has been in Justin's life since Season 1. He was (and maybe still is?) his mom's drug dealer boyfriend, who verbally and physically abused both Justin and his mom. One time, he strangled Justin right in front of his mom; she told him to stop, but only because she was worried the neighbors would call the cops. In Season 2, Mr. Porter went to Justin's apartment to look for him, but ended up getting into a fight with Seth on the lawn. When Mr. Porter was released from the police, he learned that Seth was brought in every other week. So suffice to it to say, Seth is a bad dude.
Even with Justin's mom MIA, Seth is still after Justin in Season 3 because he stole a thousand dollars when he left home at the end of Season 1, and then more when he returned in Season 2. The events of the show have all happened within roughly a year, so it makes sense that Seth is still waiting to collect. At the end of Season 2, he was watching Justin — who had returned to town but was living with Clay — from his car.
When Season 3 picks up, approximately eight months have passed since Justin returned to Crestmont. At some point in those intervening months, Seth started following Justin around town. When Justin got a barista job at Monet's, Seth harassed him about his unpaid debt, and Justin started dealing drugs for Seth in order to repay him. That led Justin to start dipping into Seth's supply and using drugs again, getting him deeper into trouble.
Desperate, Justin went to Bryce for help, and the two of them gave Seth enough cash to (hopefully) get him out of Justin's life for good. When Bryce is found dead, Justin worries Seth is the one who killed him, and that his murder is his own fault by proxy. Having a friend who can hand over thousands of dollars without blinking an eye is handy when you owe money to a criminal who has threatened to kill you... but dangerous if that criminal sees that rich friend as a way to an easy paycheck.
Any time Seth is in Justin's life, it's not good news. He's a greedy, cutthroat criminal with no morals and a lot of time to spare. So while the investigation intro Bryce's murder largely centers around his former classmates, Seth is also a viable suspect. He's scarier than all of the Liberty High students combined.
If you or someone you know is seeking help for substance use, call the SAMHSA National Helpline at 1-800-662-HELP(4357).
If you or someone you know is seeking help for mental health concerns, visit the National Alliance on Mental Health (NAMI) website, or call 1-800-950-NAMI(6264). For confidential treatment referrals, visit the Substance Abuse and Mental Health Services Administration (SAMHSA) website, or call the National Helpline at 1-800-662-HELP(4357). In an emergency, contact the National Suicide Prevention Lifeline at 1-800-273-TALK(8255) or call 911.