Thaddeus (voiced by Danny Cooksey) was one of the three secondary villains of 2009 Japanese anime horror film Alvin and the Chipmunks: Deliver Him From Evil. He is Mandy's younger brother and Thor's older brother. Most importantly, he is also Phineas Flynn's rival.
Little is known of Thaddeus before his first encountered with Donald Walls, but it is known that he was a leader and also the only male member of a group of bikers, and two of his members were Lady Tremaine and Suzy Johnson.
In the summer of 2003, Thaddeus, Lady Treimaine (Susan Blakeslee), and Suzy Johnson (Kari Wahlgren) stop at a small convenience store at Monterrey Park, California. Thaddeus and Lady Tremaine held Candace Flynn (Ashley Tisdale) hostage while Suzy Johnson was making trouble for Isabella Garcia Shapiro (Alyson Stoner). When she has Candace Flynn's thrown wallet, she wants Isabella to ask her politely for the wallet back with her real name Susan, so Isabella does. Both Thaddeus and Lady Tremaine let Candace go. While she and Isabella get ready to leave, Thaddeus was outside, drinking a bottle of guava juice as he sees Candace and Isabella inside the car. When Candace starts the car, Thaddeus begins smashing the windshield of a car that Candace was driving. However, Candace runs over the gangs' motorcycles, causing Thaddeus to fume and shout in anger while Isabella praises Candace as they drive away.
The bikers stop at the suburbs of Riverside. Thaddeus goes to a haunted house across from Ashley Grey's house and discovers Lady Tremaine dead. When he sees Donald Walls (posing as Theodore Seville), he grabs a meat cleaver, hoping to defend himself from the vengeful killer. Theodore/Donald continues his scope out of the haunted house, only to have Thaddeus appear and start attacking him with a meat cleaver. Theodore/Donald counterattacks by hitting Thaddeus with a baseball bat. While Theodore/Donald is trying to kill Ashley Grey (Bella Thorne), she runs into the haunted house. She is suddenly met by Thaddeus, who survived the earlier fight with Theodore/Donald. Enraged, Theodore/Donald turns his attention to Thaddeus by using a meat cleaver to chops off his left hand. Ashley Grey continues running in fear as Thaddeus screams in horror after Theodore/Donald chops off his left hand and hacked him 8 times with his meat cleaver. Thaddeus' one-handed corpse was later found and eventually taken away by the paramedics.
- Younger brother of Mandy.
- Older brother of Thor.
- Rival of Phineas Flynn.