Sebastian is Helen's overbearing hippie boyfriend in C.E. Forman's The Lost Seasons series, making his only appearance in "Fireworks." It was him who stuck the nickname "Lavender" on Helen. Sebastian likes control and feels threatened by Jake, or rather, Helen's growing fondness for Jake - someone he already viewed with contempt, due to his military school background and inability to 'get it'.
In 1968/9, she was part of the group that took over the dean's office in a successful protest against Middleton College's segregationist policies.
During a protest in 1969, Sebastian wandered across Jake and Helen post-sex and, despite the "free love" thing, was still left angry by the whole thing. He tried to get revenge by tricking Jake into dropping acid; instead, it caused Helen to dump him.
Forman developed Sebastian and the other characters after viewing clips from "That Was Then, This Is Dumb."