Fritz Robinson is the uncle of Lewis, brother of Bud and Joe, brother-in-law of Billie and Lucille. He is married to Petunia and is the father of Tallulah and Laszlo. He is dominated by his wife and is very nervous around her and tries that his children don't bother her.
He and his wife are the only married couple in the Robinson family who are always getting into funny arguments.
His wife is a hand puppet, controlled by him, though his children are real. It's implied that Fritz, and everyone else in the family, either think that Petunia is real or are pretending to for Fritz's sake.
It is never explained how he had kids or started a relationship with Petunia.