Śeṣa — The Eternal Serpent-Servant of Bhagavān
Śeṣa (also Ādiśeṣa = 'the Primordial Śeṣa'; Ananta = 'the Infinite/Endless'; Śeṣaśāyin refers to Bhagavān as 'the One who reclines on Śeṣa') is the cosmic divine serpent of Viṣṇu mythology — the timeless serpent deity upon whose thousand-hooded coils Bhagavān Nārāyaṇa reclines in the cosmic ocean of milk (Kṣīra Sāgara) in His Yoganidra form (the divine cosmic resting posture). The name Śeṣa means 'the Remainder' — He is what remains when everything else is withdrawn at the end of a cosmic cycle; He is also the 'Remainder' in the sense of being eternally left over, existing from before and after all creation.
In Śrī Vaiṣṇava theology, Śeṣa is the supreme icon of śeṣatva — the condition of being entirely śeṣa (= 'remainder, residual, that which exists only for another'). Śeṣa serves Bhagavān as mattress, pillow, umbrella, throne, footwear, garments — every part of his being is dedicated to Bhagavān's comfort and kaiṅkarya. This totality of self-offering, where not one fiber of Śeṣa's being exists for his own purpose, is the ideal the Āchāryas hold before every jīva: the ātman is śeṣa to Bhagavān, just as Śeṣa is. The doctrine of ātma-śeṣatva (the soul's nature as śeṣa of Bhagavān) is one of the three foundational truths of the Rahasya Trayam.
In hagiographic tradition, Nammāzhvār — the crown of the Āzhvārs — is identified with Ādiśeṣa: Nammāzhvār's divya-mangala-vigraha (sacred icon) shows him seated under a tamarind tree at Kurukūr, sheltered by the expanded hood of Ādiśeṣa. Similarly, tradition identifies Ādiśeṣa as the previous birth of Rāmānuja — who came into the world to transmit the Āchārya wisdom and theology as Ādiśeṣa came to transmit Bhagavān's teaching (Ādiśeṣa is the cosmic first teacher, described in the Yoga Sūtra tradition as Patañjali). Ādiśeṣa/Śeṣa thus spans the highest of the cosmic (Bhagavān's bed in Vaikuṇṭha), the devotional (Nammāzhvār), and the theological-historical (Rāmānuja).