Grace was a woman Sean was involved with at one point.
Having attracted her attention with the 'Sean Special'- where he pretended to be crying to appeal to a woman's maternal side-, Sean claimed that he was suffering from some unspecified-yet-terminal illness (Intending to claim later that he was on an experimental drug treatment that succeeded).
To account for the presence of the other members of the gang at his flat, he claimed that Ash was his personal trainer, Albert his financial advisor, Emma his secretary, and Mickey his PA (Although it was unspecified what Sean claimed he did for a living).
How the relationship ended is unknown, but since Sean approached another girl at the episode's conclusion, it suggests that things with Grace ended.