Nimos ... the best way to put this is that he replaces the Muse's original character, but that's not totally accurate. The fantasy aspects of RB have flipped around: before they existed only in the main character's mind. Now the mythological world is as real as everything else, and it as though she is the only one who really accepts this.
Nimos is a satyr, purebred to the hilt, and possibly the last living one in the world. He was born three thousand years ago in what is now modern Arcadia. All he remembers of the day he lost his way is darkness and a long tunnel. Unfortunately, this doesn't explain much of how he wound up three millennia later.
He and Nikki meet under strange circumstances, and get along fairly well. Before they really have a chance to connect, Nimos, having grown up very sheltered and not quite understanding just how big the world really is, runs off on his own to find his home and the rest of his family. He turns up a year later on Nikki's doorstep, a little wiser about his situation.
While it was mostly chance that led Nimos back to Nikki, he was adamant about finding her, as she was the only human he really trusted and having found none of his family or any of his kind remaining, he stands by her almost stubbornly as a friend, lover and surrogate family. He becomes extremely fond of her, despite her frigid nature, and relishes his role as her partner and backup along her many misadventures.
If Nimos were to walk along the street on a sunny day, everyone around would see plainly that he had goat's legs and horns. It's not something that only Nikki perceives, unlike the Muse had been. However, this tricky goat has a few tricks up his sleeve ... one of his abilities is to affect the perceptions of people within a certain distance, to make then believe he appears human. This is much easier if he's wearing clothes, which he hates. He's also very musically adept, though he doesn't have the patience the keep it up for long. His main ability is increased sexual stamina and an elevated pheremone level, which helps him attract an abundance of female company. He has excellent eyesight, hearing and a doglike sense of smell, and can run faster and longer than most humans. He's boisterous, immature and very affectionate, and often quite naive, though at times he displays a simplified yet startling intelligence.