Fans of the hit sitcom will know that Sheldon (Jim Parsons) had a first roommate, Sebastian (Oscar nominee Steven Yeun), prior to Leonard (Johnny Galecki) moving in.
It dictates what Leonard has to eat each day of the week, what TV shows he is allowed to watch, whether he has a girlfriend stay over, that he has to drive Sheldon everywhere he wishes to go, whenever he wishes. It even dictates when Leonard is allowed to use the bathroom.
Sheldon never met Leonard, Howard, and Raj until they were all around 24. Penny didn't move into the building until Sheldon and Leonard were both 27 and have been living there as roommates and best friends for three years; Penny was only 21 then.
The Staircase Implementation - The Big Theory (Season 3, Episode 22) - Apple TV.
It was the one titled The Staircase Implementation. He moved in with Sheldon in 2003.
A Roommate Agreement is a written contract between roommates that outlines their rights and obligations while living together. This agreement includes house rules, maintenance duties, restricted behaviors, and more. For a Roommate Agreement to be useful, everyone sharing the household must agree to it.
Sheldon and Leonard live together out of convenience not for financial reasons. Sheldon has said in the past that he wouldn't have a roommate if he could afford the rent. Penny on the other hand who lives alone on a smaller wage, is seen always struggling to get by, living paycheck to paycheck.
My take is that at the time when he was interviewing for a roommate, he was just freshly starting out at his job so the rent was too expensive for him to afford alone, but as his work in the university gradually secured him a steady income years later, he earned more than enough to live alone, but he simply chose not ...
The reason she was never mentioned in TBBT was because she didn't exist for the first 11 seasons of TBBT and was only a minor character who appeared in three episodes of YS season 2, which aired at the same time as TBBT season 12.