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Leonard is a physicist from Caltech that rooms with Sheldon in their rundown apartment building, in which the elevator always seems to be out of order. He keeps a whiteboard in their living room on which he works on theoretical physics. While Sheldon's genius seems to be focused primarily in math and science, Leonard is insinuated to be proficient in literature and history as well as the sciences; he is able to solve a woman's crossword puzzle in seconds, the answers being mostly literature based, e.g Vladimir Nabokov. It is also insinuated that Leonard's IQ is not as high as Sheldon's; Sheldon scoffs at Leonard's white board theories as common enough to be "scribbled on the wall of every men's room at MIT".

Leonard feels at home with his genius colleagues, but wants more socially. He is reluctant to let Penny know about his recreational activities (such as Klingon Boggle) and is probably reluctant to even partake in such activities in the first place, because although he secretly likes them, he doesn't like to be seen as a nerd. He also dislikes his friends' habit of rattling off different trivial facts, something to which all the other geniuses enjoy both listening and participating. Sweet but sarcastic, Leonard also seems to be the most well-adjusted one of the group; he is quick to try new things apart from his nerd friends and, between he and Sheldon, is the only who feels that he "should do something" when Penny begins to recount her recent break up.

Despite his attempts to reject his nerd status, he certainly is one, with his Star Wars hygiene products and his ability to blurt out the wrong things in front of women, such as when he's trying to invite Penny to lunch and mentions that curry is a natural laxative. He falls head over heels for Penny within seconds of spotting her and while at first it seems like lust rather than love, by the end of the pilot episode he has already announced to Sheldon, "our babies will be smart and beautiful." Sheldon, however, is quick to add, "not to mention imaginary."

(wikipedia.org)