Over time, several supporting characters have been promoted to starring roles: Leslie Winkle (season 1 recurring; season 2 regular; season 3 guest), a physicist colleague, and, at different times, a lover of both Leonard and Howard; Bernadette Rostenkowski-Wolowitz (season 3 recurring; season 4-present regular), a microbiologist, Howard's fiancée and subsequent wife, and former part-time waitress alongside Penny; neuroscientist Amy Farrah Fowler (season 3 guest; season 4 recurring; season 4-present regular), who joins the group after being matched to Sheldon on a dating website; and comic book store-owner, Stuart Bloom (seasons 2–5 recurring; season 6-present regular), who replaces Howard as Raj's best friend when Howard is in space.