Kevin came out of no where after Milo thought when he'll got held back, he'll still have his friends. Milo, Oscar, and Bea didn't know who he was. Kevin claims he has gone to all the adventures with Milo, Bea, and Oscar, but since he got held back a grade, no one ever recognizes or sees him, which made Milo realize he has to study to not lose all his friends. Milo, Oscar, and Bea still forgot who he was and he swam away crying ("Fail Fish"). He is the only person on the show to have the name Kevin (a regular name).
Even though he claims he was held back, the adventures he went on with Bea, Oscar and Milo happened in Mr. Baldwin's class.
He is more likely in the 9th grade since he is claimed to be held back.