Susan is a boil on Leela's bottom, specifically on her rump. It looks like a miniature human head (with purple hair), is conscious, can talk and likes to sing. The boil is apparently part of Leela's mutation.
Leela mentions that she has had Susan as long as she can remember and that she lances her every few months but she always grows back.
It is unknown if Susan remembers being stabbed continually by Leela over the years, or if she suffers boil brain damage from the puncture wound.
- She is based on the Britain's Got Talent contestant Susan Boyle.
- Despite being referred to as a she, Susan has a man's voice.