Lord Nityananda is the eternal associate of Caitanya Mahaprabhu, the Supreme Personality of Godhead. Rarely is the name Nimai (Caitanya Mahaprabhu) taken without that of Nitai (Lord Nityananda). Mahaprabhu cannot be approached or understood without the mercy of Nityananda Prabhu, who is the cardinal guru of all the universes and serves as an intermediary between Mahaprabhu and His devotees. He is the Lord’s active principle in both creation and lila. He is the second body of the Lord, manifesting as Balaram to Sri Krishna, Lakshman to Sri Ram and Nityananda Prabhu to Caitanya Mahaprabhu. All other forms and expansions of the Lord emanate from this second body. Nityananda Prabhu is thus the source of Sankarshan, all the Vishu’s, and Ananta Sesa. As Vishnu tattva He and Advaita Acarya are worshipped in the same category as Caitanya Mahaprabhu.(http://www.atlantaharekrishnas.com/content/view/245/40/)
Advaita Acarya, although Visnu Himself, was in the mood of a Vaisnava. As a Vaisnava, He felt compassion for the fallen conditioned souls who were suffering without Krsna consciousness. Although previously Navadvipa had been a great center of devotion, it had deteriorated into a center of dry learning. Thus, Advaita Acarya felt compassion for the fallen souls of the age, so engrossed in material affairs, devoid of devotional service to Krsna. In the mood of a humble Vaisnava, although He was Visnu Himself, He felt that He was not capable of delivering the fallen souls. He believed that only Sri Caitanya Mahaprabhu Himself could preach the yuga-dharma of hari-nama-sankirtana and deliver them.
Mahek, Simren, Ishaan, Tulsi, Janki, Sneha, Manushi, and other sources