Jefferson the Mad Hatter is a recurring character in Once Upon a Time. He is the roleplay character of MasteroftheHat.
Jefferson works as a portal jumper with the power of a special hat and has the ability to travel from world to world. After procuring items at Rumplestiltskin's behest, he shows up at the Dark Castle while the Dark One's pupil, Regina, is attempting to learn magic. He goes to Rumplestiltskin and presents him with the crystal ball, to which the Dark One responds he specifically requested for the slippers. Jefferson informs him that the slippers were moved and only the globe was obtainable, and asks if he wants it or not. Rumplestiltskin assents, and tells Jefferson to take whatever gold he wants from the floor. When Rumplestiltskin tells Regina to leave his presence and exits the workroom after stressing there is no way for her to resurrect Daniel, Jefferson comes up behind and talks about a man, a wizard of sorts, from another world who possesses the power to bring back the dead. In return for bringing the wizard, Regina as queen must grant him a royal pass to travel through her kingdom without needing permission, which she agrees to.
Jefferson returns with Dr. Frankenstein, and goes with him and Regina to Daniel. When Dr. Frankenstein performs his examination and declares he needs a strong heart to perform a successful procedure, Regina confronts Jefferson, but he reassures and asks her to hear the doctor out. He then goes with her and Frankenstein to Cora's mansion, where they enter the vault of hearts. After Dr. Frankenstein takes one, he sets up an operating table inside a tent to get to work. Jefferson and Regina wait outside as he attempts to bring Daniel back to life, but it seemingly fails.
In actuality, Jefferson is in on a scheme with Rumplestiltskin and Dr. Frankenstein. In a deal for Dr. Frankenstein to obtain a heart for his own use, Rumplestiltskin wanted Regina to move on from her wish of bringing back Daniel and give into her dark side. As things are accomplished since Dr. Frankenstein feigned a resurrection attempt, he receives the heart and escorted home by Jefferson via portal jumping.
Some time passes, and Jefferson retires from being a portal jumper. Instead, he lives a meager life with his daughter, Grace, and sells mushrooms. One day, they have a playful game of hide-of-seek in the woods. When they arrive at home, they discover the Evil Queen's carriage outside their house. The Queen wants Jefferson's help to retrieve something, and tells him that if he cooperates, Grace will want for nothing, but Jefferson refuses. While at the market the next day, Grace wants a rabbit doll, but Jefferson cannot afford it. His daughter is not upset, but he begins to worry that he cannot meet all of her desires, unaware that this incident was orchestrated by the Queen.
Jefferson and Grace return home to enjoy a pretend tea party with an improvised toy rabbit he made. Deciding that he should try to make her life better, Jefferson asks Grace to stay with the neighbors while he performs the task the Queen desires from him, promising to return later, despite his daughter's pleas against it.
He later visits the Queen and offers his help. Using his hat, Jefferson opens a portal on the ground, to which he and the Queen jump in. They reach a room inside the hat with various doors to many different worlds. Jefferson warns if two people enter the door to Wonderland, only the same amount of people can exit out. They enter through a looking glass and run into the Caterpillar, who puffs smoke rings in their faces, prompting annoyance from Jefferson.
They arrive at a hedge maze which leads to the Queen of Hearts' palace, however, the Evil Queen makes short work of the magical maze by sending a ball of flame through it. They manage to get what the Queen wants, which turns out to be her precious father, Henry, and escape from the Queen of Hearts' guards. Before they leave, the Queen reveals she plans to take her father back to the Enchanted Forest, thus, Jefferson cannot go back. She entraps him in place and disappears into the looking glass with her father.
The guards catch and take him to the Queen of Hearts. She beheads him, yet he is still alive. She tells him that if he wants his body back, he must tell her what happened with him and Regina. He does and he is given his body back. He is told that he needs to make another hat to leave. Jefferson is later seen making a hat, having gone mad trying to get it to work, and the room is shown to be filled with thousands of hats.