Iśāna Ṭhākura is counted as one of Caitanya Mahāprabhu’s branches:

Śrīnātha Miśra, Subhānanda, Śrī Rāma, Iśāna, Śrīnidhi, Śrī Gopīkānta, Bhagavān Miśra [were branches of Caitanya’s tree]. (Caitanya Caritāmṛta 1.10.110)

Iśāna Ṭhākura¹ was Jagannātha Miśra’s household servant. Only an eternal associate of the Supreme Lord could have the good fortune to render Him personal service in His own home. The Lord accepts the service of His own confidential associates. When Mahāprabhu was born, Iśāna had the chance to take the Lord on his lap and to hold Him affectionately. He tolerated all of little Nimāi’s childhood pranks and answered His every whim. Nimāi was unable to go without Iśāna for even a moment.

How can I understand the activities of Iśāna Ṭhākura, whom Nimāi used to lovingly call “father”? The son of Śacī, Nimāi Cānda, was his very life and Nimāi would not go anywhere without him. In His childhood, Nimāi was very naughty and would make all kinds of capricious demands that Iśāna would do his best to fulfill. (Bhakti-ratnākara 12.95-7)

Iśāna Ṭhākura also had the chance to serve Nityānanda Prabhu, Śrī Caitanya Mahāprabhu’s other self. The Lord would plead with His mother to be able to invite Nitāi and upon her consent would go Himself to invite Him. Before the two sat down to eat in Śacī’s house, Iśāna Ṭhākura would bring water to wash Their feet. (Caitanya Bhāgavata 2.8.59)

As one of the Lord’s eternal associates, Iśāna Ṭhākura was fully aware of devotional truths. He knew that the worship of the devotee is more important than even the worship of the Lord Himself. Thus he performed all kinds of service in the house of Śacī Devī, who is none other than Yaśodā Devī and was blessed by her affection. The Lord also blesses one through His devotee’s merciful attitude. The Lord’s mercy follows upon the mercy of the devotee.

Iśāna Ṭhākura’s glories are described in the work known as the Vainṇava-vandanā:

vandiba īśāna-dāsa kara-joṛa kari
śacī-ṭhākurāṇī yāre sneha kaila vaṛi

I pay obeisances to Iśāna Dāsa with folded hands, for Śacī Ṭhākurāṇī was very affectionate towards him. (Bhakti-ratnākara 12.94)

The brāhmaṇa said, “I am just returning after visiting Iśāna. What can I say, no one really understands his great qualifications. He has the reputation of knowing all the truths about the Lord, and he served Śacī Devī by doing whatever she needed. Because he rendered such constant service to the Lord’s mother, Iśāna is uniquely fortunate in the fourteen worlds. Even though I saw with my own eyes the affection she had for him, I am unable to find words to describe it.” (Bhakti-ratnākara 12.90-3)

Iśāna, who was most fortunate in the fourteen worlds, served the Lord’s mother constantly. (Caitanya Bhāgavata 2.8.74)

When Mahāprabhu took sannyāsa and left for Purī, Iśāna was left with the responsibility of taking care of His mother Śacī and His wife Vinṇupriyā Devī and supervising their household. In his commentary to the Caitanya Bhāgavata (2.8.73), where it is said that Iśāna Ṭhākura did all the cleaning of the house and later took the remnant prasāda, Śrīla Bhakti Siddhānta Sarasvatī Gosvāmī Ṭhākura writes: “Iśāna was the servant in Mahāprabhu’s house. He gathered up the rice that had been thrown all over [by Nityānanda Prabhu] and made the house proper. There was no limit to Iśāna’s good fortune. He rendered service to the Lord’s mother throughout his entire life. Even after the Lord took sannyāsa, he was engaged in the service of the Lord’s mother and wife, showing the extent of the blessings he had received. He is therefore described as ‘most blessed’ or ‘the most fortunate among the blessed.’”

Iśāna Ṭhākura lived a very long life. He was still living even after the disappearance of Vinṇupriyā Devī and all the other devotees of Mahāprabhu in Navadvīpa. Thus when Śrīnivāsa Ācārya, Narottama Ṭhākura and Rāmacandra Kavirāja came to Navadvīpa on pilgrimage, only Iśāna was able to show them where all the places of the Lord’s pastimes were. In the Bhakti-ratnākara, it is said that when Iśāna showed these devotees the holy places, they were already old and in a state of disrepair. This gives some idea of how old he must have been.

Nearly everyone has disappeared from Navadvīpa. No one but the Lord’s servant Iśāna was still living. (Bhakti-ratnākara 11.721)

Śrīnivās Ācārya, Narottama Ṭhākura and Rāmacandra Kavirāja received Iśāna Ṭhākura’s blessings in Navadvīpa-dhāma and were embraced by him before leaving to visit Srikhanda where they met with Raghunandana Ṭhākura. When they arrived there, they received the news that Iśāna had left the world.

I heard from some people while on my way here that Iśāna has disappeared. (Bhakti-ratnākara 13.21)

After Mahāprabhu and Śacī Mātā had disappeared, Vaṁśīvadanānanda Ṭhākura served Vinṇupriyā Devī and Iśāna Ṭhākura.

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¹ Other than this Iśāna, there were several other associates of the Lord who had the same name.

1. Sanātana Gosvāmī’s servant was named Iśāna.
2. There was a devotee in Vṛndāvana of that name. He was a member of the party of devotees who accompanied Rūpa Gosvāmī to the house of Vitthaleswar to have darśan of Govardhana-dhārī Gopāla. He also is said to have blessed Śrīnivāsa Ācārya, Narottama Dāsa Ṭhākura and Śyāmānanda Prabhu when they started their voyage to Bengal with the Gosvāmīs’ books.
3. In the Gauḍīya Vaiṣṇava Abhidhāna, there is mention of a certain Iśāna Ācārya who is identified as Vraja’s Yauna-mañjarī.
4. The author of the Advaita-Prakāśa was named Iśāna Nāgara.