|For the real Leo Fitz, see Leo Fitz|
Manifesting from Leo Fitz' Stress
The Doctor originated from Leo Fitz's long stay in the Framework, in which Fitz's dark side was fully unleashed and he acted as one of HYDRA's leaders. The Doctor first manifested in the real world due to a conjunction of Fitz's old brain injury, his trauma regarding his actions in the Framework, lack of proper sleep, and stress due to struggling to find a solution to contain the Fear Dimension in the Lighthouse. Initially, Fitz thought that The Doctor he was seeing was an illusion generated by the Fear Dimension, but it soon appeared that Fitz actually suffered from a major psychic split which caused him to switch personalities, with The Doctor stating that they had to remove Daisy Johnson's Inhuman Control Device, no matter how painful and dangerous it could be.
This version of Fitz who had experienced the psychic split died in the Battle of Chicago, but Jemma Simmons, who embarked on a quest to find another version of Fitz still frozen in space knew that the love of her life struggled with this dark persona still trapped in his mind and that she should do her possible not to unleash it again.
Revenge on Jemma Simmons
The Doctor once again appeared to Leo Fitz and Jemma Simmons in a simulation generated by the Cerebral Fusion Machine in which Fitz and Simmons had been put by Atarah. Gathering a group of HYDRA soldiers, The Doctor quickly confronted them in a recreation of the Lighthouse, claiming that they could not escape from him before ordering his men to shoot them down.
The Doctor was later able to lure Simmons by pretending to be the real Fitz in danger. He then had his soldiers capture her and put her into a replica of the Theta Brain-Wave Frequency Machine. The Doctor taunted her, explaining that he would take whatever memories of Simmons he found interesting before killing her, and mocked Fitz's weakness and Simmons' attempt to control her own pain. He then activated the machine and proceeded to torture Simmons. However, he was eventually attacked by a simulation of Quake, who used her powers to take down both The Doctor and his soldiers.
The Doctor kept chasing Fitz and Simmons, finding them locked inside a Containment Module with Id Simmons also threatening them as she had previously captured Fitz. Initially, The Doctor expressed nothing but distrust towards Id Simmons, ordering her not to interfere with his project, but he soon took an interest in her. By the time Fitz and Simmons were able to sort out their issues, The Doctor and Id Simmons gave in to their mutual feelings and began passionately making out and engaging in sexual intercourse, much to Fitz's and Simmons' bewilderment. The simulation was ended soon after, causing The Doctor to disappear.
The Doctor was a manifestation of Fitz's darkest aspects which had been highlighted by the Framework. As such, he was cold, ruthless and sadistic, and did not hesitate to resort to torture to achieve his goals, like he did to Jemma Simmons. He was also determined to get revenge on Simmons for the death of his father, and felt nothing but disdain towards the real Fitz, who he deemed weak because of his emotions.
Due to being an incarnation of Fitz's dark side, The Doctor felt attracted to the dark side of the love of Fitz's life, Id Simmons, and they quickly expressed their mutual attraction upon their first encounter.
- Genius Level Intellect: Just like the real Leo Fitz, The Doctor was an immensely clever scientist, although his intellect could be expressed through violent means like torture and psychological manipulation.
- Theta Brain-Wave Frequency Machine: In the simulation created by the Cerebral Fusion Machine, The Doctor put Jemma Simmons into a replica of this machine in an attempt to extract her memories.
- Alistair Fitz † - Father