Who is Agni?
Wikipedia: Agni is a Hindu deity, one of the most important of the Vedic gods. He is the god of fire (energy) and the acceptor of sacrifices for onwards conveyance to other deities. The sacrifices made to Agni go to the deities because Agni is a messenger from and to the other gods. He is ever-young, because the fire is re-lit every day, and is immortal.
In the Rig Veda (I.95.2), a Rishi prays - for the ten eternal powers to bless Tvashtr (the supreme mind which creates all things) with the birth of Agni which is a reference to the ten undisclosed powers that nourish Agni.
The word "agni" is Sanskrit for "fire." Agni has three forms: "fire," "lightning" and the "Sun."
Sthaulāśthīvi says that Agni is the drying agent which neither wets nor moistens anything. Śakapūni says that the word Agni is derived from three verbs – from "going," from "shining or burning," and from "leading."
Yaskacharya says that it is called Agni because it is Agrani, the forward leader who is the ever awake disseminator of knowledge and the first principle of thought which manifests as speech; it is carried at the front in all ritualistic undertakings (yajnas).
Who Worships the God:
The Vedic people developed the worship of Agni and personified and deified Agni as the sacrificial fire, as the priest of the gods and as the god of the priests, who through yajna carries the oblations to the gods, the celestial controllers of the mysterious and potent forces of nature, to ensure the continuance of conditions favourable to mankind.
As far as Vedic deities are concerned, Agni occupies, after Indra, the most important position. In the Rig Veda, there are over 200 hymns addressed to and in praise of Agni.
In Hindu scriptures, Agni is the God of Fire, and is present in many phases of life such as honouring of a birth (diva lamp), prayers (diva lamp), at weddings (the yajna where the bride and groom circle the fire seven times) and at death (cremation).
Agni is also worshipped as the symbol of piety and purity; as expression of two kinds of energy (light and heat), and he is the symbol of life and activity. When Agni is worshipped all gods (gods are symbols of life) lend their powers him.
In Vedic Astrology, Agni is the bodily fire which is represented by Mars; resides in the human body as the bodily warmth, the human body is their common vehicle. The seven arms or tongues of Agni signify the seven rays of the sun, the seven colors of the solar spectrum, the seven days of the week, the seven chakras of the body and the seven planets. The South-east direction is ruled by Agni who represents bodily heat and knowledge and is the prime cause of birth.