Kevin is often seen as Dr. Mephesto's silent, but faithful lab assistant. In "An Elephant Makes Love to a Pig", he helps Mephesto breed a pot-belly pig and an elephant. In "Starvin' Marvin", Chef is inspiring the town to help solve the turkey crisis, when he asks Mephesto what Kevin is, who replies by saying that it wasn't an important matter.
During "Cartman's Mom is a Dirty Slut" and "Cartman's Mom is Still a Dirty Slut", Kevin's name is revealed, and he assists Dr. Mephesto in finding out who Cartman's real father is. In "Cartman's Mom is a Dirty Slut", when announcing the potential candidates for Cartman's father, the cliffhanger narrator refers to him as "that little monkey guy who follows Dr. Mephesto around".
He later reappears in subsequent episodes, usually alongside Dr. Mephesto. Until "201", where he screams in distress, Kevin was entirely silent.
Kevin's story is told in the song "Mephesto and Kevin" by Primus from Chef Aid: The South Park Album. He was apparently a failed attempt to create a perfect pop singer, a test tube baby created from Michael Jackson's sperm and the egg of an unspecified, musically talented female donor, brought to term inside the womb of a llama. It is also revealed Kevin is legally Dr. Mephesto's son.
Kevin looks similar to Dr. Mephesto in that he wears black shoes, brown pants, and a yellow floral-pattern shirt. He has black hair, similar to Fosse McDonald, and whiskers. He is often described as a gopher-boy.
- "An Elephant Makes Love to a Pig" - Seen helping Dr. Mephesto.
- "Starvin' Marvin" - Asked about by Chef.
- "Cartman's Mom is a Dirty Slut" - Seen helping Dr. Mephesto; also a prospective father of Eric Cartman.
- "Cartman's Mom is Still a Dirty Slut" - Seen helping Dr. Mephesto.
- "Prehistoric Ice Man" - Seen helping Dr. Mephesto unfreeze Larry.
- South Park: Bigger, Longer & Uncut -
- "Spontaneous Combustion" - Seen helping Dr. Mephesto bring to light Randy Marsh's false science.
- "Jakovasaurs" - Seen helping Dr. Mephesto learn about Jakov.
- "Are You There God? It's Me, Jesus" - Seen helping Dr. Mephesto give hormone pills to Stan Marsh.
- "Cartman Joins NAMBLA" - Seen helping Dr. Mephesto combine a chipmunk and provolone cheese.
- "Freak Strike" - Kevin cameos during the freaks song.
- "I'm a Little Bit Country" - Seen with Dr. Mephesto during the final song.
- "It's Christmas in Canada" - Seen in the crowd with Dr. Mephesto.
- "Goobacks" - Kevin has a cameo during the final song.
- "201" - Seen helping Dr. Mephesto.