Humans make fickle worshipers. Bigger and better gods were completely forgotten, while Hermes / Mercury, a member of the supporting cast in his own time, now has a planet, a chemical element and his caduceus is all over the place, used as a symbol of commerce and various commercial organizations.
The modern fame is not undeserved, as the messenger god was quite hard-working in ancient times, being in charge of... well, mostly everything – trade, thieves, inventions, borders, crossroads, travelers, poets, orators – on second thought, throw in all writers and literature as well, shepherds, athletes and athletic competitions, measures of all sorts – oh, and all these, while crossing to the Underworld every now and then, since he was one of the selected few with this ability.
The Roman Mercury is a copy of the Greek Hermes, and did not evolve from a local character, though speculations have been made that he might be linked to the Etruscan god Turms. Since we know so little about the Etruscan gods, we can link them anyway we want. Instead of morphing from one god, Mercury emerged from several – the so-called Dei Lucrii, a group of minor Roman deities of commerce, trade, and other lucrative activities.
The Romans made some efforts to welcome Mercury and include him in their legends, the most detailed of them being the one told by Ovid in his Fasti, according to which Mercury fell in love with a nymph (why not, everybody else was doing it) called Lara or Larunda, and their children were the Lares – extremely important Roman household deities. Among other things, the Lares were, just like Mercury, deities of borders and crossroads.
By another nymph, Mercury is sometimes credited as being the father of Evander, the man famous for having a small hut on the Palatine Hill long before Romulus and Remus were even born. In his Amphitryon, Plautus depicts Mercury as nothing more than Jupiter's servant.
Mercury's temple was located close to the Circus Maximus, fittingly in a commercial area, between the plebeian and the patrician neighborhoods, suggesting his position of mediator, but at the same time, outside the city sacred inner walls, indicating that the cult at least, if not the god himself, was a foreign one.