Emily Sweeney came to the show rather late in the game, but she actually became a pretty major character for her few seasons. She was okay but she definitely was not in my absolute favorites.
If I remember right, she was a dermatologist that Raj was set up on a date with and he blows his chance but ends up getting a second date by luck.
If I remember right, she was probably his longest lasting relationship. But they never really seemed that right as a couple.
She was also into some scary dark kind of stuff. A lot of guys are into freaky girls but there are moments where I thought she might be a serial killer. Eventually her relationship with Raj goes sour and she leaves the show.
She later shows up with all of Raj’s exes for a reunion/feedback meeting. She was not alone with not regretting breaking up with him.