Susan is revealed to be a vegetarian in "The One With The Dozen Lasagnas". She works in advertising.
Susan and Carol had an affair while Carol was married to Ross. Ross and Susan do not initially get along very well, mainly because Ross resents Susan for "stealing" Carol away from him. Susan resents Ross for his romantic history with Carol as well as his awkward behavior around her. In "The One With The Sonogram At The End", they want to give Ross and Carol's unborn child the surname Willick-Bunch, excluding Ross from the title. During Carol's labor in "The One With The Birth", Ross and Susan even manage to compete in who fetches Carol some ice chips, resulting in Carol banishing them both from the delivery room. During their argument in the janitor's closet, the two agree with Phoebe's musing that Carol is giving birth to a baby that has three parents that truly care for it. Spurred on by this and the name tag on the uniform Phoebe wears, the two decide to name the child Ben.
Carol and Susan announce their plans to get married in "The One With The Lesbian Wedding", but Carol's parents refuse to attend the wedding, leading Carol to doubt her decision. Ross initially hesitant to see his ex-wife remarry, finds himself in the position of being the one to encourage her to go ahead with the ceremony despite her parents' opposition. At the reception, Susan thanks Ross for his part in saving the wedding, and offers to dance with him; he first declines the offer, but later agrees. Apparently, this resolved their strained relationship.
Susan's father appears to be in the military and both her parents seem very supportive and accepting of her relationship with Carol, specifically stating that while there is a Mother's Day, following her wedding to Carol.
Carol and Susan celebrated their anniversary together (although not the wedding anniversary) in "The One Without The Ski Trip".
Her final appearance was in the alternate reality episode of Season 6 where it is implied that no matter what Ross and Carol would not end up together. The mothers, Carol and Susan, are still together today in many other episodes and stories.
In a 2019 interview, the actress Jane Sibbett (who portrayed Carol) believed Carol and Susan had moved to London and frequently worry about Ben while he is in college (source). As to compensate that fear, they adopt children and home-school them. 
- "The One With The Sonogram At The End" (First appearance)
- "The One Where Underdog Gets Away"
- "The One With The Dozen Lasagnas"
- "The One With The Candy Hearts"
- "The One With Two Parts, Part 1"
- "The One With The Birth"
- "The One With The Breast Milk"
- "The One With The Lesbian Wedding"
- "The One Where Old Yeller Dies
- "The One With The Metaphorical Tunnel"
- "The One Without The Ski Trip" (Mentioned only)
- "The One With Rachel's New Dress"
- "The One Where Phoebe Hates PBS" (Mentioned in cut version/appears in uncut version)
- "The One That Could Have Been, Part 2"
- "The One With The Holiday Armadillo" (Mentioned only)
- "The One With Rachel's Date" (Mentioned only)
- "The One With Monica's Boots" (Mentioned only)