Ernest is a bully who commissions sports that involve kids misbehaving (in unambiguously bad ways, rather than resisting oppression by adults, which is a central theme of the series). Ernest is always seen with a lollipop in his mouth and is frequently called "Ernie" by Numbuh 4, a close associate and nemesis of his.
Ernest first appeared in Operation: M.A.T.A.D.O.R., where he holds the "Bully Fights", in which innocent adults are driven mad with caffeine (being tricked into going there by signing up for a "How to be a Better Dad" seminar) and used for Bull Fighting. Numbuh 4 competed in the sport as a matador, which upset his team, as it went against their moral policies and had been interfering with his duties as a KND Operative.
While the team was out battling Soccer Mom, Numbuh 4 went to the Bully Fight stadium to discover that his opponent this time was his own dad. Numbuh 4 then realized the error of his ways and set all the captured adults free, putting Ernest out of business and defeating Soccer Mom in the process of leading them away from the bullies. Interestingly, the adults set free by Numbuh 4 and his dad were seen with the kids that were captured by Soccer Mom, implying that she and Ernest had been working together in some way.
Ernie appears again in Operation: B.R.E.A.K.U.P., where he is the commissioner of the game "Vaseball", a version of Baseball where fragile glass and pottery items are used as balls. He secretly plans to inform the parents of the kids who play his game of their misbehavior and get them grounded, so that there would be no one to tattle on him when he vandalizes every house in the neighborhood with eggs and toilet paper.
Numbuh 4 competed in the sport to retrieve his mom's stolen vase and, when he found out about Ernie's evil plan, had to stop it from being smashed. Numbuh 4 eventually defeated Ernie, but in the process destroyed every object in the storeroom, and everyone involved, minus Numbuh 4, ended up being grounded, including Ernie himself.