Cobain is a boy close to Stewie's size that Lois arranges a playdate with when they try to keep of the facade of being young parents in "Young Parent Trap".
Having given up their old house to Meg and Chris to move into the Millennial Village apartment complex, Lois leaves Stewie on the complex playground with his new 'playmates'. Despite being ready for an afternoon nap, he discovers that none of the other kids nap or even have organized bedtimes. As a result of this and his all natural diet, Cobain reveals he's never grown and is actually age six.
During a later play session in the pool, the kids are abandoned by their parents and Stewie discovers that they've all drowned, leaving him to try to fish them out of the pool by himself.
Cobain is voiced by Issac Ryan Brown.