Reuben is Gantu's sidekick and Jumba's 625th experiment. His best friends are Tank and Amnesio who aren't confirmed to make physical appearence in the series.
He was first seen beting with Gantu at the begining and then at the end chating with Tank and Amnesio.
Reuben ready to throw the bowl with spagetti to Gantu's head
Reuben chating with Amnesio and Tank