Reuben is the bridegroom at the wedding. He only appears on screen for about a second, pulling his bride, Miss Starter, to safety. He is a cousin of Ron, in what way is not specified, but since the wedding is specified as "the Stoppable-Starter wedding" he is from the father's side.
Reuben is the husband of Miss Starter, the nephew to Mr. Stoppable, and cousin to Ron Stoppable, he now has a new adopted baby cousin called Hana Stoppable. His relationship to Ron's other cousin, Shaun, is unclear.