Dr. Alex Sartorius was the co-founder of Stagg Enterprises.
Specific details on Sartorius' past prior to co-founding Stagg Enterprises was unknown, although he had been brought up in the Roman Catholic Church before he gave up faith for science (which hinted at his being a lapsed Catholic), and that he had pyrophobia.
Twenty years before the events of Scarecrow's attack on Gotham, he met Simon Stagg, and proceeded to form the pharmaceutical company, Stagg Enterprises. However, as he didn't care about the limelight or the financials, Sartorius let Stagg name the company after himself as well as let him take the credit for various breakthroughs that the company made, including a cure for the Tugela Virus and a treatment for Collier's Syndrome, that Sartorius was actually responsible for discovering. Sartorius also trusted Stagg with the financials, due to Stagg stating he didn't need to worry about them. Sartorius continued to believe it until the year of Scarecrow's attack, when he learned that the financials for Stagg Enterprises had a huge hole in them, and caused him to suspect that Stagg had lied to him all that time.
Sartorius was then forced to let Stagg develop bioweapons and chemical weapons, which was enough of a problem as there weren't very many legal buyers for them, as well as Stagg deciding to break the law by abducting several thousand humans to act as test subjects for the proposed weapons, despite it being unethical, illegal, and life-threateningly dangerous, with Stagg only assuring him that the abducted people "won't be missed." In addition, he also ended up forced to let Stagg sell bioweapons and chemical weapons to not only the US Army, but also to its enemies, with the latter party that had a far larger percentage. The illicit bioweapons sales became so prevalent that Sartorius even noted that Stagg Enterprises had become a pharmaceutical company in name only. The last straw for Sartorius was Stagg accepting a business deal with Scarecrow, and worked with him to modify one of his inventions, the Cloudburst, from what was intended to be an inoculation device for many to a weapon of mass destruction. After he decided that he had enough of his friend's unethical actions, Sartorius then made a recorded confession, and also acted as the first time that he had ever entered the limelight. During that time, Sartorius also developed some anti-paranoia pills, which he eventually succeeded in creating by mixing anti-anxiety pills with whiskey. Sartorius also made clear in his recording that he had no intention to garner any sympathy or redemption for his role in Stagg's actions, and only intended to expose his former friend.
Unfortunately for him, however, Stagg became privy to Sartorius's actions behind his back, and had a militia soldier bring him before him under the pretense of wanting to have a "talk" with him. Stagg accused Sartorius of treachery, and made him a lab rat in the Fear Toxin as his punishment. As a result, Sartorius hallucinated that he was consumed by fire.
- Unlike in the comics, Dr. Alex Sartorius did not become Doctor Phosphorus. However, his identity as him was given a subtle nod in the Arkham Knight City Story "Lab Rat", which revealed Sartorius' fate and had him hallucinate that he was constantly on fire due to the effects of the fear toxin that he was exposed to by his former friend.