Joachim had the appearance of a man with red eyes and pale blue hair, however his true hair colour is later revealed to be stark white as a result of the ingesting of Gnosis. As a medical professional. Joachim wore a maroon collared shirt with a brown waistcoat and yellow neck decoration, beneath a medical white coat.
His appearance as the executive priest of the D.G cult is much more elaborate, boasting colourful robes of teal, magenta, gold and cream. He also carries around an ancient staff used the the alchemists of Crossbell's past.
Joachim initially has the semblance of a lazy, but polite and helpful man. He aggravates his colleagues by his frequent fishing trips (He is an enthusiast and a member of the fisherman's guild). When he reveals himself, he still seems to keep the facade of being a reasonable and logical person.
However, beneath the facade lies a great deal of fanatical insanity, complete with a devotion to what the cult consider their God. He will stop at nothing, and use every extremity available to him to complete his cult's desires, including endangering and experimenting on children, enslaving masses of people and conducting large scale crime. After probing into Joshua Bright's memories, he acknowledges Georg Weissmann as a person he would get along with. Joachim shows a complete disregard for human feelings and emotions and is easily driven to even greater depths of insnaity from his consumption of great quantities of Gnosis.
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. When most of the cult was exterminated in various multi-organisation efforts across the continent, Joachim managed to hide small remnants, although this was unknowingly also part of a greater plot by the true manipulators behind the scenes, the Crois Family.
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