Kashchey

One of the many "Ageless", the Deathless Black Snake had lived for a very long time – long enough to witness the true nature of Terra and the various past conflicts involving several now-defunct nations, and likely achieved this feat 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.

Throughout the Arknights timeline, the Deathless Black Snake is known to have two bodies: Kashchey and Koshelna; their true or original body, however, remains a mystery although it is suggested that the Deathless Black Snake's true/original body is serpentine in nature due to their name.

(Source: Arknights Wiki)