Maddox was first introduced in Know Thy Enemy. He was revealed to be working with Isobel Flemming, who was compelled by Klaus Mikaelson, in a diabolical plan to capture Katherine Pierce, Alaric Saltzman, and the moonstone. After kidnapping both Alaric and Katherine, Maddox later informed Isobel that she her role in the plan was complete, which triggered Klaus' compulsion and forced her to kill herself by removing her daylight amulet and burning up from exposure to the sunlight. Maddox later cast a spell that allowed Klaus' spirit to possess Alaric's body so he could infiltrate the Mystic Falls Gang.
In The Last Dance, Maddox advised Klaus on a method to kill Bonnie, as she had harnessed the magic from the witch burial ground; he cast a protection spell on Klaus/Alaric's body and encouraged Klaus to trick Bonnie into attacking him until she eventually died from overuse of magic.
In the episode Klaus, Maddox, with the help of another witch named Greta Martin, performed a spell that returned Klaus' spirit to his own body, as Klaus believed Alaric's body was too vulnerable. In The Last Day, both Maddox and Greta were assigned in capturing Caroline, Tyler, and Jules to be sacrificed in the ritual to break Klaus' hybrid curse. To do this, Maddox used magic to telekinetically throw Carol Lockwood down the stairs in order to lure Tyler and Jules back to Mystic Falls. While guarding the prisoners inside the tomb, Maddox and Damon became involved in a violent confrontation, during which Matt arrived and shot Maddox with his rifle, incapacitating him long enough for Damon to snap his neck and kill him.
Maddox, like Greta, was a witch who was extremely loyal to Klaus. He would use his magic to aid in Klaus whenever asked, such as the spell that transferred Klaus into Alaric Saltzman's body, as well as the spell that protected Alaric's body from most physical harm. If Klaus demanded it, Maddox would deliberately hurt or kill innocents, as demonstrated when he attacked Alaric and Carol.
Maddox was a young man in his 20s, with dark hair and a pale face that nearly always wore a stern expression. He always dressed in a preppy manner, frequently wearing a black coat.
Maddox is of Welsh origin, and the meaning is "Good; generous"
- Maddox is a Welsh name, meaning "son of Madoc". It gained popularity when Angelina Jolie adopted a baby with this name in 2003. It is the pen name of writer George Ouzounian, who got internet fame with The Best Page in the Universe.
- "My character is a guy named Maddox, and he's a witch. It's kind of fun, because I basically hold up my hand and people go flying off walls." -Gino Anthony Pesi about his character. 
- Maddox was the first Caucasian witch to appear in the TV series.
- Maddox served as Klaus' right hand prior to Klaus acquiring Stefan and then his hybrids.
- Since Maddox was a witch and was thus supernatural, his spirit went to the Other Side after he died. Following the collapse of the Other Side, Maddox most likely found peace or was sucked into oblivion.
- Since he worked personally with Klaus, we can assume he is a very powerful witch.