In Megatokyo, backstory is usually explained through dialog of the characters. Erika does not discuss herself or her history with anyone, so all the information we have on her is from sources that are either prejudiced, incomplete, or in some cases fanciful.
Erika seems to have become an entertainer either right out of, or even while she was in high school. Kimiko knew her since this time (probably she went to the same high school), and remained to be her friend while Erika's career took off. And Erika's career did take off. She had several hit records, and she was the voice actress for the lead role of Moeko in a hit anime series Girl Phase. Her career was at its height about three years ago when all of the sudden, she came on stage at a concert, announced to the fans that "Everything they loved was a lie" and quit.
At one point, Kimiko explained to Piro what happened. According to her, Erika had a relationship with Hitoshi from the time she was in high school. No one but Erika knew what she saw in Hitoshi, least of all Hitoshi. As her career grew and grew he became more and more lost in the shadows of career, until one day he left her. Distraught that everything she has worked for in her career had only taken something else immeasurably important to her, she abruptly quit the industry.
The early days of her retirement seem to have been filled with anoymous lovers, alcohol and resentment, until two years ago (or, as Kimiko put it, later, "her life fell apart"). By the time the story starts, she seems to have calmed down, but she is still prone to anger and responds to invasions (whether perceived or actual) of her personal space with brutal and direct methods.
One of the mysteries of the story is what happened to Erika's money. She is not hurting for cash, and seems to cover many of Kimiko's expenses, yet she has "Booth Babe" jobs and works as a clerk at MegaGamers. Another mystery is why she is still employed at that store, as her work ethic leaves a great deal to be desired. There are speculations that she might in fact own it.
While her discussions with Kimiko make clear she is not celibate, she has no current romantic interest, and her attitude toward men is dismissive ("all men are fanboys") and contemptuous. More recently, Largo has taken her seriously, which she seems to respect.