Mitra's relationship with Varuna is so close that he is only addressed in one hymn (iii. 59). Due to the scarcity of information in that hymn, his distinct character appears somewhat indefirite.
Pass on the Karma!
Multiply the positivity, through the Sanatan
He marshals men and keeps an unblinking eye on the tillers while uttering his voice. He is the great Aditya who marshals the people, yatayati, and the epithet yatayáj-jana arraying men appears to be peculiarly his. Savitr (i. 35) is identified with Mitra because of his laws, and Visnu (i. 154) is guided by Mitra's laws: statements indicating that Mitra controls the path of the sun. Mitra is produced by Agni, who goes ahead of the dawns (that is, is kindled before dawn), and is Mitra when kindled. Mitra at sunrise is contrasted with Varuna in the evening in the Atharvaveda, and Mitra is associated with day in the Brahmanas.
The conclusion drawn from the Vedic evidence that Mitra was a solar deity is supported by the Avesta and Persian religion in general, where Mithra is unquestionably a sun-god or a god of light specifically associated with the sun.
The name's etymology is unknown, but it must have originally meant 'ally' or 'friend,' because the word often means 'friend' in the RV., and the Avestic Mithra is the guardian of faithfulness. Because the kindly nature of the god is frequently mentioned in the Veda, the term must have been applied to the sun-god in his aspect of a benevolent power of nature at first.