Lord Nityananda, who is Balarama Himself, the first direct manifestation or expansion of Krsna, is the original spiritual master. He helps Lord Krsna in His pastimes, and He is a servant of the Lord. Lord Caitanya is the same Lord Krsna, and Lord Nityananda is Lord Balarama. Lord Nityananda fulfills all of Lord Caitanya's desires. The source of the sentiment of servitude is indeed Lord Balarama, and the plenary expansions who follow Him are all influenced by that ecstasy.
Sri Rama, or Balarama, is the protector of the devotees of the Lord. Baladeva acts as the spiritual master of all devotees, and by His causeless mercy the fallen souls are delivered. Balarama comes with Sri Caitanya Mahaprabhu, and both of Them are so merciful that in this age of Kali one may very easily take shelter of Their lotus feet. They come especially to deliver the fallen souls of this age. Their weapon is sankirtana, hari-nama. Thus one should accept the sword of knowledge from Krsna and be strong with the mercy of Balarama.
Paraphrased from text and purports of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.
"Narottama dasa Thakura says that Balarama and the son of Maharaja Nanda have advented Themselves as Gaura-Nitai. To propagate this fundamental principle, we are establishing a Krsna-Balarama temple to broadcast to the world that worship of Gaura-Nitai is the same as worship of Krsna-Balarama.
Although it is very difficult to enter into the Radha-Krsna pastimes, most of the devotees of Vrndavana are attracted to the Radha-Krsna lila. However, since Nitai-Gauracandra are direct incarnations of Balarama and Krsna, we can be directly in touch with Lord Balarama and Lord Krsna through Sri Caitanya Mahaprabhu and Nityananda Prabhu. Those who are highly elevated in Krsna consciousness can enter into the pastimes of Radha-Krsna through the mercy of Sri Caitanya Mahaprabhu. It is said: sri-krsna-caitanya radha-krsna nahe anya. Sri Krsna Caitanya Mahaprabhu is a combination of Radha and Krsna.
Balarama means guru-tattva. Balarama represents guru. Yasya prasadad bhagavat-prasadah. If we want to understand Caitanya Mahaprabhu, if we want to understand Krsna, then we must take shelter of Balarama. Nayam atma bala-hinena labhyah. This bala-hinena labhyah, this Vedic injunction, means "Without the mercy of Balarama you cannot understand, you cannot realize your spiritual identification. So that Balarama comes as Nityananda Prabhu. Balarama haila nitai. Therefore we must take shelter of Balarama."
Lecture by Srila Prabhupada, 02-02-77, Bubaneswara
Bhaktivedanta Book Trust.