What we know so far about Matt:
".. Danger, Beth..."
Matt's a stoner, and has a reputation for being dangerous.
Matt somehow knows where Beth lives.
Matt also knows Beth's mobile number.
Sammy is freaked out by Matt, but we don't know why... "Leave Beth alone"
Matt likes Beth, and also used to date Tara. As Petra tells Beth: "But he's so into you - it's weird how history repeats... Matt and Tara, Matt and you..."
Matt was the last person to see Tara alive.