'Deathless Black Snake'

Gender: ~!Male (as Kashchey)
Female (as Koshelna)!~
Race: Ageless
~!Pythia (as Kashchey)
Hippogryph Liberi (as Koshelna)!~

The 'Deathless Black Snake' is an NPC in Arknights and one of the antagonists of the Main Theme.

One of the many "Ageless" who are present during the various past conflicts involving several now-defunct nations, the Deathless Black Snake likely achieved their immortality through an ability where the Deathless Black Snake implants some of their consciousness into the victim's psyche, allowing them to suggest or influence others into agreeing with their ideals over time. ~!This was evident when it's revealed that Talulah was aware of Kashchey's presence when she was possessed by him.!~

~!Throughout the Arknights timeline, the Deathless Black Snake is known to have two bodies: Kashchey and Koshelna.!~

(Source: Arknights Wiki, edited)