Doug is an ex-boyfriend of Penny's. ("The Fuzzy Boots Corollary")

Leonard found Penny saying good night to Doug when he tried to return her mail. Doug asked Leonard how he was and called him "bro" which Leonard returned. During Leonard's misunderstood date with Penny she told him about Doug. He was a quick fling of 36 hours of guilt ridden sex she had while still getting over her former live-in boyfriend Kurt.

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