An imaginary first cousin of Sheldon that he created to cover up a lie why he and Leonard couldn't come to a performance of Penny in the episode "The Loobenfeld Decay". He tells Penny that Leo escaped rehab and is currently in Motel 8 at Long Beach and Leonard will drive him to meet up with the entire Cooper family for a drug intervention.
According to Sheldon, his first cousin Leopold Houston, known as Leo or Lee, is 26 years old and has a Facebook site, an online blog describing his descend into drug addiction and a listing on eHarmony.com.
He was born in Denton, Texas and brought up as a navy brad on varying military bases around the world.
This has led to an identity as a constant outsider that never fitted in and drove him into drug abuse. Sheldon also describes him as the "quintessential middle child" that was driven into drug addiction to compensate his unattended need for attention. In addition, he has a genetic predisposition to inadequate serotonin production.
The latter is questioned by Toby Loobenfeld, who thinks that this would be "pop psychology" and that the real reason for Leo's drug abuse is that he was abused by a Naval Chaplain in 1992 on the Philippines.