Ādiśeṣa (lit. “primordial remainder“; also Ananta, “the infinite“) is the great serpent who serves as the eternal couch (śayyā), the umbrella (chattra), and the sandals (pādukā) of Bhagavān Nārāyaṇa in Śrī Vaikuṇṭham. He is counted foremost among the nitya-sūris — the eternally liberated beings who have never undergone bondage and who exist solely for kaiṅkarya to Bhagavān. Ādiśeṣa's thousand hoods spread above Bhagavān as a royal canopy while he reclines in yoga-nidrā upon the Milky Ocean.
In the Purāṇas, Ādiśeṣa is the lord of the serpents (Nāgarāja), who bears the weight of the earth upon his head, and who in the Rāmāyaṇa age incarnated as Lakṣmaṇa — the younger brother wholly devoted to Śrī Rāma. Patañjali, the author of the Yoga Sūtras and the great grammarian, is also traditionally identified as an incarnation of Ādiśeṣa. In Sri Vaishnava hagiography, however, the most celebrated avatāra of Ādiśeṣa is Rāmānuja: born in Śrīperumbūtūr, he spread the sampradāya across the subcontinent and authored the Śrī Bhāṣyam, just as Ādiśeṣa eternally holds Bhagavān aloft.
The association is affirmed by Rāmānuja's very birth: his long arms (ājanubāhu) and the hood-like shape of his head are cited in the tradition as physical signs of his serpentine origin. Maṇavāḷa Māmunigaḷ's Upadēśa Ratnmālai hymns Rāmānuja as the very embodiment of divine grace descended to rescue bound souls — a mission wholly in keeping with Ādiśeṣa's identity as the eternal servant who makes Bhagavān accessible.