Toby Cavanaugh is Jenna's step-brother and somewhat of a shady character. One night before Alison's disappearance, Alison, Emily, Aria, Spencer, and Hanna were in Emily's room changing clothes, when Alison claimed she saw Toby looking at them from the window. Alison then had the idea to get revenge on Toby by playing a prank on him that involved fireworks. The prank went wrong and ended up blinding Jenna by mistake. When the police came Alison blackmailed Toby and told him that if he took the blame for them, that she would keep his secret (which is still unknown at this point). Toby was sent to a reform school after the inccident and returns back to the girls' school at the same time Aria returns to the states. Emily starts to build a friendship with him due to being lab parthners, but seems to still harbor some uneasy feelings about him- but those may not be the only feelings she has for him.
Btw. He is really hot :)