I would like to expound upon this:
Unbeknownst to many, the Shinigami of the Reaper Death Seal is a woman:
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The Reaper is a Hannya Demon who are all females and solely females.
The Shinigami is also a priestess holding a prayer beads and reciting a prayer before commencing the sealing ritual:
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The Reaper's initial stance with it arms spread is a clerical/priestly stance:
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Like Rikūdou/Hagoromo's stance:
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It was established and well known by now that the Shiki Fūjin required Rikūdo/Hagoromo's Tablet as the catalyst for the Shinigami Mask to unseal the Reaper:
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It is because the Shiki Fūjin is Rikūdo/Hagoromo's Jutsu
and the Reaper is most probably Kaguya Ōtsutsuki, Rikūdo/Hagoromo's mother:
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As to why Kaguya became the Shinigami of the Shiki Fūjin, Kishimoto has not yet explained or revealed why, either as an end effect of eating the Shinju's fruit or as a mother's sacrifice to aid her son in fighting the Jūbi.
The Hannya Mask (般若) takes it's name from the Sino-Japanese word for prajna or wisdom, the mask was named as such because the Perfection Sutras (Hannya Scroll) were effective against demons.
The Prajnaparamita or the Hannya Scroll speaks of the Six Perfections to become a Bodhisattva aka a Sage, a Sennin
The Six Perfections Are:
1. Dāna pāramitā: generosity
2. Śīla pāramitā : virtue
3. Kṣānti (kshanti) pāramitā : patience
4. Vīrya pāramitā : diligence
5. Dhyāna pāramitā : contemplation
6. Prajñā pāramitā : wisdom, insight
He who has followed the Six Perfections becomes a Bodhisattva, a Sennin.
The Bodhisattva then has mastery of the Six Realms/Paths also known as Rokudō 六道
1. Naraka Path
2. Preta Path
3. Animal Path
4. Asura Path
5. Human Path
6. Deva Path
Much like Rikudou Sennin who is the Sage of the Six Paths:
