Mitra has a cousin, and the relationship is one of the securest results of comparative Indo-Iranian philology. Avestan Mithra is the same name from the same root with the same core meaning, the covenant. The two traditions then went different ways with him: in India Mitra stays paired with Varuna and gradually recedes, so that by the classical period he has almost no independent cult, while in Iran Mithra grows enormously, acquires a great hymn of his own in the Avesta and a place in the calendar, and is later the starting point, at some distance and through disputed channels, for the Roman Mithras of the bull-slaying reliefs. The atlas carries the Iranian and Roman figures as separate entries, which is the right treatment: shared descent is not identity, and the Roman cult in particular is a Roman creation whose debt to Iran is still argued over.
Inside India the pairing itself became a theoretical object. Georges Dumezil built a large part of his comparative account of Indo-European sovereignty on Mitra and Varuna as two complementary faces of rule, the contractual and the magical, the priestly and the terrible. The reading is influential and it is contested in roughly equal measure, and it is worth knowing that it is a modern framework applied to the hymns rather than a scheme the hymns state.
What the Rigveda itself asserts is narrower and firmer. The two are invoked together, they uphold rta, they see what is done, and the offender is released by their favour and not otherwise.