Joel Kaplan was the father of Olive Kaplan, an infant clinic patient in the Season 1 episode Babies & Bathwater. Joel is married to Rachel Kaplan, Olive's mother. Joel was portrayed by actor Michael Simpson.
Joel and Rachel brought Olive to the clinic when she was lethargic and suffering a fever. Gregory House soon diagnosed Olive with pneumonia and malnutrition. When he discovered Joel and Rachel were trying to raise Olive on a vegetarian diet, he admonished them as vegetarian diets don't always provide all the fat and protein needed by infants. Olive needed to be admitted, but Joel and Rachel promissed to change Olive's diet right away.
However, after Lisa Cuddy learned about the malnutrition, she reported the matter to child services, who had Joel and Olive arrested and took guardianship of Olive. House protested violently as he believed the parents were "stupid", not neglectful. However, Cuddy countered that House was required to report any suspicion of neglect to child services, and Edward Vogler was looking for an excuse to fire him.
Joel and Rachel came back to Dr. House. He tried to avoid them, but they swore to him that Olive's diet had been recommended by a university educated nutritionist, Rachel's uncle. They also informed him they fed her whenever she was hungry. Dr. House was intrigued and agreed to run more tests.
Dr. House had correctly guessed that Olive's low weight wasn't the result of malnutrition as he suspected, but a malfunction of her thymus gland. He informed Cuddy who agreed to call child services to vindicate Joel and Rachel.