All the characters have a very defined style. As proven in this Big Bang Theory Art. All we see are the clothes, yet we know which characters are which.
Leonard Hofstadter is my "boyfriend" from the show. I would definitely give Penny a run for her money in the pursuit for Leonard's heart. His style consists of straight legged pants, a hoodie, more often than not a army green jacket,a ironic or comic related tee, Converse, and his black rimmed glasses. We love it, we know it, and it helps define his character. It relates to us that he likes classic with his Converse, fun nerdy things via his tshirts, and comfort via the rest of his outfit. It makes him lovable.
Johnny Galecki (the actor that plays Leonard) has a very different style. Johnny is a bit more sexy. Often with his wavy hair slightly unkempt, a bit rakish. He has style. He likes a nice pair of jeans or trousers that are cut well, a good jacket, and he likes to wear v-necks. You can find many images of him showing a little chest with a v-neck or a button down unbuttoned a little. He prefers roughed up boots or a classic oxford shoe with a twist. He wears scarves and leather jackets. When he is dressed up he chooses well cut suits and usually has a unique color or fabric.