Joachim Guenter was born in S.1170.
He studied at the Medical University in Remiferia. He ultimately moved to Crossbell to start working at the St. Ursula Medical College as an associate professor. All this time he stayed in touch with his former classmate, Professor Seiland, by sending her regular letters.
An associate professor by day, Joachim served as one of the last, if not only remaining, high priest of the D∴G Cult, which denies the existence of Aidios. As a result of taking Gnosis, Joachim lost the ability to sleeping which was why he was capable of working as a professor during the day and fulfilling his duties as a high priest at night. The drugs also turned his hair from the original grey colour to light blue.
During the Cult Incident in Crossbell, it is revealed that many of the strange occurrences in the state were (in)directly caused and manipulated by Joachim. He was Ernest Reis's physician and prescribed him Gnosis, which led to Ernest's abnormal obsession with the mayoral position of Crossbell to the extent of an assassination attempt on Mayor Henry MacDowell.
It is also revealed that he handed Gnosis to the Boss of Revache, Marconi, who in turn handed this to Preston, then-commander of the Crossbell Guardian Force, in the form of a 'supplementary nutrition'. Indirectly, this gave Joachim control over thousands of troops.
When he was finally corned by a collaboration between the Special Support Section and Bracer Guild, he was labelled as "more heretic than Red Constellation" by Randy Orlando, "no longer human" by Elie MacDowell and "even more disturbed than Weissmann" by the Brights. Before the battle, Joachim's blue hair colour returned to its original grey.
Joachim starts off the first fight with some alchemical archaism assistance, but is eventually defeated, so he uses the Evil Eye to immobilize the group that Weissmann used, based on the memories of Estelle Bright he was able to perceive through the Gnosis. Lloyd asked Joachim if he was responsible for killing his brother, upon which he realized that Lloyd was the younger brother of Guy Bannings. He stated that he had hired Revache to assassinate him as he was proving to be troublesome with his investigation, but confirmed Marconi's statements that Guy had been already killed by someone else before the act could be carried out.
Lloyd then yelled that they would protect KeA from someone like him, causing Joachim's concentration to falter, shattering the Evil Eye. Realizing that he is outmatched, he takes an overdose of Red Gnosis pills, transforming into a massive demon.
At the height of the battle, he extends his demonic arms and thrusts them through the floor toward the SSS and bracers, capturing them for a final attack. He is stopped, however, by Renne and Pater-Mater. Once the others are freed, they defeat him for good. Due to his overdose of Red Gnosis, Joachim does not revert back into a human, but regains his consciousness, which he reluctantly thanked them for. He states that even though their accomplishments may be unfulfilled, their ambitions have been realized, and that KeA will play a role in it all before he dissipates.
In Ao no Kiseki it is revealed that Joachim had successfully killed the SSS after transforming but KeA reset the timeline allowing the SSS to resolve Renne's problems, resulting in her rescuing them from what would have otherwise been their death.
Zero and Ao no Kiseki