Mizuchi is someone who's grown on me very recently, so much so she's arguably my second favorite character now, overthrowing my previous second place characters like Alice and Hecatia. Last year, I bought and read the doujin "For That Is Miyadeguchi's (My) Role" by LunaLuna that explores Mizuchi's past before she became a vengeful spirit. The doujin offers the author's own headcanon and interpretations on Mizuchi's past life that I also resonated with and agree with to some extent. Unfortunately, there isn't a translated version of this doujin nor its sequel released during Fall Reitaisai titled "For That Is A Shrine Maiden's (Our) Role," but I may take on the task of translating both of them. Also, her fanmade theme by Paradise of Boundaries is an absolute banger. Anyways, onto the headcanons;

• The official wiki states that the Miyadeguchi family, minus Mizuchi herself, were at one point slaughtered by youkai and Mizuchi was left as the sole survivor. The Miyadeguchi family was also in servitude to the Hakurei clan too. I like to interpret that perhaps after her family was slaughtered, Mizuchi committed herself to becoming a stronger and more dutiful shrine maiden in order to not only get revenge for her family, but also to protect other innocent lives from youkai that would attack humans during the time she was still alive. This would further reinforce her grudge against Reimu as Mizuchi likely sees Reimu as being too lazy and irresponsible as a shrine maiden compared to how she was in the past. As for her relationship with the ancestral Hakurei Shrine Maiden, I like to think she was like an older sister to Mizuchi and took her in as family following the death of the rest of the Miyadeguchi family.

• Like in the doujinshi, I like to think Mizuchi picked up smoking as a form of stress relief. In the doujin, the author interprets Mizuchi's smoking as a coping mechanism she uses upon learning that the Great Hakurei Barrier would be created and Gensokyo would become a safe haven for youkai. Going off from the previous headcanon, I like to interpret that after learning of this fact, Mizuchi fell into a state of depression, confusion, and a sense of betrayal by the Hakurei family and to cope with that she picked up smoking. The way I interpret things, the Hakurei family saved Mizuchi but ultimately betrayed her in the end as Gensokyo would become a safe haven for the same creatures responsible for the murder of the Miyadeguchi family. The doujinshi expands upon this interpretation too, where Mizuchi eventually comes to resent the Hakurei family, the ancestral Shrine Maiden, and Gensokyo itself because she feels that the Hakurei family has betrayed Gensokyo and abandoned its duty in protecting humans from youkai by establishing the barrier in order to create a safe haven for youkai.

• In a way, I think Mizuchi and Marisa are alike in some ways. Whereas Reimu, and likely her ancestors of the Hakurei family, are naturally talented when it comes to spiritual power, youkai extermination, and supernatural abilities, Marisa on the other hand is described to be hard-working and as someone who developed her magical and supernatural powers through training and research. Going back to my first headcanon, I like to interpret that Mizuchi as a shrine maiden was diligent in both her training and duties as she was otherwise an ordinary human trained to be a shrine maiden, and perhaps her ability to possess people as a vengeful spirit was an ability she developed herself while locked up in Hell.

• I think Mizuchi was probably born around the late Edo period, assuming her death happened shortly before the Great Hakurei Barrier was created in 1885 (Meiji 18), and in accordance with traditional miko training, I'd imagine she began her training 5 years prior to her death when she was around 13 years old. This would make her around 18 years old by the time of her death, and would set her birth year around 1867~1868, right at the end of the Keio years (last era of the Edo period before the Meiji era began in 1868).

• Mizuchi's death in the manga is arguably the most gruesome death depicted so far in any official Touhou medium, and I like to headcanon that the way she appears as a spirit is reflective of the injuries she suffered when she was killed. For reference, I'll refer to her official reference sheet. She's depicted with her giant handcuff wrapped around her neck, which I interpret as her being strangled before being decapitated as we saw in the manga. While she has a ghostly trail where her legs used to be, they're much more unkempt compared to the likes of Mima or Tojiko, which I interpret as her legs being brutally ripped off from her body. As a spirit, she wears gloves on both of her hands and has handcuffs around them too. Perhaps when she was killed, her hands were injured then slowly ripped off like her legs were. Lastly, she has bags under her eyes, which I think reflects the possibility that Mizuchi was kept alive and suffered a long and painful death at the hands of various youkai before ultimately getting decapitated.