@mirewitch
As this comment is posted (20201115T051315Z), individuals in Japan seeking gender marker change that includes alteration of family registry needs to be:
1. Individual must not currently married
2. Permanently sterile through sex gonads removal, medically transitioning a.k.a HRT (administration of exogenous cross-sex hormones) doesn't count.
3. External genitalia
That means: oophorectomy but more commonly radical hysterectomy alongside metoidioplasty or phalloplasty + scrotoplasty for individuals born female, or orchidectomy alongside penectomy + vaginoplasty + labiaplasty for individuals born male.
Kamentarou might or might not pursue the requirements due to her preferences or other factors. Known reasons ranges from: those surgeries are the least significant factor in one's external presentation, realization that medicine doesn't yet provide fertility after those surgeries are taken, dissatisfaction of current medicine technology results, lengthy recovery + post operation maintenance, etc, etc.
EDIT: minor typos.
EDIT 2: forget to include which country does this code apply.