Leonard Leakey Hofstadter needed a place to stay when he started at the university, so he answered an ad and went through all of the hurdles to become Sheldon's roommate, even though he was warned at the time that Sheldon was crazy. For some reason, he didn't believe it, and chose to sign the Roommate Agreement and move into Sheldon's apartment. Leonard put a couch in the living room, so they could actually have guests, and brought in new friends (Howard and Raj), much to Sheldon's displeasure. When a new girl, Penny, moved next door, Leonard wanted to meet her. She talked him into getting something back from her ex-boyfriend, and a relationship of sorts was formed between Penny and the men across the hall. Leonard always wanted to date Penny, and eventually his dream came true. Eventually, they broke up, and Penny dated a not-too-bright guy. After spending time with her new man and Leonard, she found out she can no longer handle being with a stupid guy, so perhaps the door is open for Leonard to get back together with her? We shall see. Whatever happens there, Leonard always has his friends Sheldon, Howard and Raj to fall back on to hang out with.
Sheldon Cooper
Despite being constantly rude to people he finds less superior to him (which is just about everyone), he is often unaware of the fact his actions are off-putting, and, on several occasions, has shown that he follows a very polite protocol that he picked up from being raised in a highly religious family. These actions most times greatly surprise his friends. Also, if someone compares a situation to how a fictional character who is known for heroics (mostly characters from comic books and sci-fi movies/TV shows) would help out, he will offer his assistance, though somewhat begrudgingly. With this "Hero-Complex" mentioned, it should be noted that the most believable reason that Leonard, Howard, and Raj are his friends,despite his obnoxious behavior, is because he saved the trio's lives by removing a small bomb the three inadvertently created in Leonard and Sheldon's apartment.
Kaley Cuoco
Penny is a 20-something waitress at the local The Cheesecake Factory. She recently moved across the hall from Leonard and Sheldon, replacing an overweight transvestite with a skin condition. She grew up in Omaha, Nebraska on a farm, where she had an ambivalent relationship with her father who quite bluntly wanted a boy. Penny is characterized as someone with attachment issues and as temperamental. She becomes emotionally attached to men who only want her for a sexual relationship.
