I will not go into the life of Nichiren here. His placement in line with Nam Myoho-Renge-Kyo indicates the principle of the "Oneness of the Person (Buddha) and the Law." His name reinforces this principle (see "Nichiren in Chinese"). 

Nichi is the central kanji in the title of the Dai Nichi Kyo. According to the Shingon sect, Dainichi Nyorai (Great Sun Buddha) is the source from which all other Buddhas and bodhisattvas spring. The Dai Nichi Kyo reveals the aspect of the "Person." Dainichi Nyorai is on the Gohonzon and will be discussed later. 

Ren is the central kanji in the title of the Myoho Renge Kyo (Mysterious Law Lotus Flower Teaching). The Myoho Renge Kyo reveals the universal "Law" or Dharma. 

Nichi (Sun), and Ren (Lotus), combine to indicate the oneness of Person and Law (see also Nichiren). 

Nichiren wrote: "Names are important for all things...Giving myself the name Nichiren signifies that I attained enlightenment by myself."  

—Major Writings, Vol. 1, p. 236, "Letter to Jakunichi-bo"

The large kanji on the left (your left) side of the Gohonzon: 
Starting from the top is Dai Bishamontenno — Great Heavenly King Bishamon (Skt. Vaishravana). Bishamon protects the North. He is also known as Tanmon, which means "Listening to Many Teachings." He appears in the 26th chapter of the Lotus Sutra and pledges to protect its votaries. 

Under Bishamon is the Siddham sound character Ho (the "h" is not pronounced) indicating Aizen Myo-o (Skt. Ragaraja).  Aizen is the deity of romantic (sexual) love. He illuminates the principle that "worldly desires equal enlightenment" (Bonno soku bodai). 

Under Aizen is Dai Zochotenno — Great Heavenly King Zocho (Skt. Virudhaka).  Zocho protects the South. He should not be mistaken for King Virudhaka who lived during the time of Shakyamuni. 

The large kanji on the right (your right) side of the Gohonzon: 
The topmost character is Dai Jikokutenno — Great Heavenly King Jikoku (Skt. Dhritarashtra). He protects the East. He appears in the 26th chapter of the Lotus Sutra and vows to protect those who embrace the Lotus Sutra. 

Under Jikoku is the Siddham sound character Ham (the "h" is not pronounced). Ham indicates Fudo Myo-o. Fudo defeats devils trying to hinder Buddhist practice. Fudo means "Immovable in Fire." Myo-o is the brightness of the sun and moon. His placement on the Gohonzon indicates that the sufferings of life and death are nirvana (Jp. Shoji soku nehan). He appears in the 28th Chapter of the Lotus Sutra where he vows to protect both the Sutra and its practitioners. He holds a thunderbolt that is the Buddha speaking in his loudest voice saying "Make all things work towards enlightenment."  

As a note of interest Aizen (Ho) and Fudo (Ham) are the only true sound characters on the Gohonzon. When combined they make the sound OAM. OAM is considered a sacred sound, encompassing all other sounds.  

Under Fudo is Dai Komokutenno — Great Heavenly King Komoku (Skt. Virupaksha).  Komoku protects the West. He has divine eyesight that discerns evil. He punishes evil (so watch out) and arouses an aspiration for enlightenment.