Mitra

[In post-Vedic India], Vedic Mitra practically disappears and Varuna is relegated to a subsidiary position

when about 1350 B.C. a treaty was concluded between the king of Mittanni [...] and the Hittite ruler [...], the former calls upon the gods Mitra, Indra, Varuna, and the Nâsatya as guarantors of his sworn obligations.

As the Avestan Mitra had a luminous character, so had the Vedic Mitra.

We cannot account for Roman Mithras in terms borrowed from Persian Mitra.