Disclaimer: The timeline created below is a matter of combining both videogame lore and historical facts in relation to the Sengoku era; as well as facts in relation to the possible mental conditions. This is only a character theory timeline and is not meant to impede or force any opinions. The purpose is to help fellow writers and artists.
Doujun/Dousaku belong to Fromsoftware.
Timeline coverage: Due to the games manner of concealing time, dates, and ages— this frame will go by Comic/Manga & Game. This covers roughly a 15-20 year timeframe based on the comics description.
Meaning: Path, teachings(Buddhism) - to bloom; to come out
Age: Unknown however based on events through the game and character’s within, a reasonable estimate is between 29-35 in game.
Occupation: Former Seeker Bomb Monk(sect of the warrior monks), Ex-disciple of Master Dogen, Presently hired healer/experimentalist in the Ashina dungeons.
Quest: Located in the Ashina Dungeon Entrance.
Pre-Comic/Isshin wins the war and is Lord of Ashina
Origin- Unknown, however, based on historical facts in relation to Japanese Buddhism (specifically the Zen and Hossō Sects due to Senpou’s design inspiration). Monasteries have their own rules including how they run and ages within ranks (similar to samurai and foot soldiers). Based on average monkhood ordination, Doujun would have entered the monastery around 10 years old and would become a novice monk. He would have also partook in nanshoku with a nenja(elder monk) to learn religious teachings as well as the rank or specialty that was expected.
Theory: One can Assume Doujun’s mentor’s name was Saku. Otherwise this mentor has been remained nameless.
Based on attire, Doujun was taught and trained to be a warrior monk as the art of a Seeker/Bombthrower.
Study: Buddist Monasteries, despite religious discrepancies would have a small group of monks who practiced in “Buddhist Healing” with Kampo influences. Not only did they heal fellow monks, but were also hired by courts and locals to heal and perform medicinal research.
Assumption: Based on Doujun’s Kimono colouring, he was one of the few in that group.
During the Comic:
Important Lore Note for this section: Appearance note outside of what the timeline goes over: Unlike the other monks at Senpou Temple, Doujun’s pupils are still apparent with a steel silver hue and not “glassed over” or has a “Clouded Silver appearance.” Meaning he did not become host to the larvae despite the monks forced consumption of them(unknown if it was physically eaten, injected ect ect)
Reasoning: Due to the Monks consumption of centipede larvae.
Based on:
* In the infested text, it's clear that having "worms" is a blessing to the monks and being "chosen by the worms" (which can be assumed is a larvae being able to parasite the host and grow). The parasitic insects attack/feed off properties that make up melanin(organism pigmentation) causing the loss of eye/skin pigmentation ....Then when they are "blessed" as the Centipede grows it obviously will need other nutrients and take advantage of the host completely.
Regarding Ape and Hainbei: Matured centipedes can skip the development stages of the Larvae like the Guardian ape, therefore feeding into the host itself.
* Not only are the monks glassy eyed but so is the Great Carp. If you visit the great carp corpse when you kill it, the larvae resides where its stomach was. Either the Carp had been fed the insects or the were in Fountainhead from the start.
Baby centipedes are described as being worm-like.
Due to the comic status is ongoing (7/15/2019) not much can be written regarding this section except Doujun’s trauma and eventual mental split.
Lore: The Senpou monks had been stealing away children for decades to use in the trials of recreating the Dragon’s heritage and in turn making a Divine Child of Rejuvenation for immortality purposes.
Theory/Thought: Doujun, trained as a seeker would be more or less out in the field “spiriting away children” being oblivious to what was going on in experimental sanctums as well as main hall. Based on his innocence later on upon Dousaku deceiving him, it can be believed that the children really were being taken to the temple grounds to perform "virtuous deeds."
Isshin’s Discovery: Upon the lords discovery of the monks encouragement in the teachings of immortality— corrupting the Buddhist beliefs to worshiping the centipedes, Isshin barricaded and refused the monastery, referring to them as degenerates.
At this point the secrets have spilled and to Doujun’s horror not only discovers the truth in regard to the children but also discovers the Temple has turned their backs on Buddha in their search for immortality and use of the Rejuvenating Waters— specifically witnessing the corruption of the centipede.
Doujun & Dousaku
Fact: In order to fully understand the trauma the character went through, you need to step into what we know. This character has a severe case of Dissociative identity disorder.
Definition: Dissociative identity disorder (DID) is a diagnosis characterized by having two or more distinct people, each with his or her own identity and personality, that alternately take control over a person. This disorder may be caused by intense trauma, such as ongoing physical abuse, sexual assault, and/or emotional/psychological abuse/event that occurred at a younger age. It’s believed to be the brains coping mechanism for the victim to escape of leave the traumatic memory/situation.
Note: The development of the personality does not happen immediately at the time of the event but can occur some time afterwards depending on the victim.
It is clear Dousaku becomes the personality that takes on the role of Doujun’s former life with the Senpou monks beliefs and as they slowly warped it into an obsession for immortality. His strategies and teachings align with the corrupted beliefs of the monks.
Doujun’s self is left as a naive but clever personality, doing whatever he can as a student to eagerly please his masters. to become an enlightened disciple of medicine.
It can be determined Dousaku left the Temple in fear of what the temple had become.
Mentions in-game that can be assumed to be prior to the game's timeframe
Lore: Doujun was a Disciple along with Emma to the great Master Dogen. Dogen’s teachings are speculated to be leaning more towards scientific healing and discovery.
During the time in which Doujun was under Master Dogen’s studies is when Dousaku’s personality fully developed and “emerged” and convinced Doujun that the other physician was mad and expressed extreme jealousy against Dogen. (this can be a reference to how medieval Physicians and Buddhist heals often would rival one another)
Since Dousaku has claimed himself as the superior personality, it can be assumed he was the one to be hired or set to task by Ashina nobles to work within the dungeon, experimenting on creatures in the hopes of discovering ways to better assist Ashina in winning the war over her lands.
Based on the reaction of betrayal Doujun expresses to Dousaku during their quest, it can be conceived that he was easily manipulated into thinking he was searching for a “treatment” with the rejuvenating waters sediments.
In-Game
This is a part in which I do not feel needs to be heavily touched on but will make small mentions.
It is made evident by the time Okami reaches Doujun, he is beyond helpless, fully consumed into Dousaku’s manipulations. Over the time of the quest it can be seen Doujun starts to come to a realization that he has not only been fooled, but is in a position in which he can not escape.
Reasoning: The consistent need to confide to Okami over his doubts & lack of enthusiasm towards Dousaku and upsetting disbelief to the first experiment he performed in the questline.
The consumption/Injection of Red Carp eyes and the effects:
Reasoning: The descriptions of the Red lump, Red-eyed Carp, Doujun's conversations of his experiments and questline as well as the description of creature/individuals who have red eyes. Also the fact the Demon of Hatred ends up with red eyes and Isshin’s explanation of Shura.
"Those with Red Eyes are dangerous and filled with rage that is difficult to control through the power of man alone. They do, however, fear the flame above all else."'
"Red Eyed Carp are considered incomplete creatures, Carp with only eyes that are red cannot become “masters”."
Theory: Red eyes are not only an after effect of those whom have consumed some form of stagnated rejuvenating water but ALSO who are consumed by a sense of failure/incompletion/defeat that turns into an endless rage causing their eyes to glow. Thanks to The experimentations of Doujun/Dousaku it's noticed that despite being experimented on, the person you send to him will not have red eyes until after you receive the bloodstained letter.
Note: Based on how much of a victims madness of their misfit circumstances has consumed them will determine the glow intensity until they are too far gone to come back from madness.
This concludes that in the despite Doujun’s fears, he felt that he not only failed himself but his master as well. This is why in the end he desperately in a frenzy attacks you, for the sake of failing his master.
Feel free to DM me if you are interested in the sources I used, any questions or feedback :)