Śeṣī — The Supreme Lord in the Śeṣa-Śeṣī Relationship
Śeṣī (Sanskrit: from śeṣa = remainder/that which belongs + -in suffix indicating possessor; 'the one who owns the śeṣas') is the correlate of śeṣa: if the jīva is the śeṣa (the one who belongs), then Bhagavān is the śeṣī — the supreme master to whom all jīvas and indeed all of creation eternally and inseparably belong. This is one of the foundational relational concepts of Viśiṣṭādvaita ontology.
The Śeṣa-Śeṣī Relationship as Cosmic Structure: Rāmānujāchārya's doctrine of śarīra-ātma-bhāva (body-soul relation) provides the metaphysical grounding: just as the body exists entirely for the soul, all jīvas and the material universe exist entirely for Bhagavān's delight (Bhagavān's ānanda). Bhagavān is therefore śeṣī in the most comprehensive possible sense — not merely the owner of some beings, but of all beings without exception. This is not an external ownership (as a landlord owns property) but an intimate, interior ownership (as the soul owns the body — the body is constituted for the soul's purposes from the inside out).
Śeṣī and the Praṇavam: Piḷḷai Lokāchārya's analysis of the praṇavam (Om) in Mumukṣuppati reveals the śeṣa-śeṣī structure encoded within it: the akāra (the syllable a in Om) directly denotes Bhagavān Nārāyaṇa as the Supreme — the akāraṃ is the pradhāna vachanārtham (primary denotation) of Bhagavān as śeṣī. The entire jīva-Bhagavān relationship is thus inscribed in the first syllable of the Veda.
Ādiśeṣa as Model: Ādiśeṣa (Ananta), Bhagavān's divine serpent throne and Nammāzhvār's celebrated form, is the paradigm of perfect śeṣatva — complete surrender of existence, action, and purpose to Bhagavān. Ādiśeṣa's every function (as seat, canopy, footwear, etc.) is purely for Bhagavān's pleasure. The Śrī Vaiṣṇava sampradāya contemplates Ādiśeṣa's relationship to Bhagavān as the ideal toward which all jīvas aspire: to be as completely Bhagavān's as Ādiśeṣa is.