Dewey is Hal and Lois' fourth son. He is Malcolm, Reese, and Francis's youngest brother, and often falls victim to their pranks. Dewey is very intelligent and musically gifted. Despite his intelligence, he is placed in a remedial class for slower students (or "Buseys") due to a misunderstanding. Dewey remains in the class and serves as their self-appointed teacher. By the seventh and final season of the show, the Busey class is no longer mentioned(Possibly due to the constant help his classmates had, they no longer needed to rely on him as they once had) and eventually he was transferred to the Krelboynes without interference from both Reese and Malcolm.. He is the youngest member of the family until Lois gives birth to Jamie. He was acted by Erik Per Sullivan.
- Dewey had an ear infection.
- Has been known to hold grudges for a very long time, especially with Hal, Reese and Malcolm. Lois and Francis are the only ones who he hasn't hold grudges with.
- Unlike Malcolm, Lois having acknowledge his musical talents, intends for Dewey to be rich and happy. Because according to her, he was always meant to be happy and also the only one who appreciates her efforts.
- Worked hard to ensure Jamie was never ignored.
- Like Malcolm and Jamie, Dewey is intelligent and gifted. Francis is intelligent as well, but is also a rebel and Reese is the only one who is dumb.
- Daisy is Dewey's counterpart in If Boys Were Girls and had a close relationship with Lois, Renee and Mallory. However, she soon showed a devious side to her and was the only one who told Lois off that they're not above using her tactics against her because they're girls.