The reason for the rating on this can be summarized by a character study:
Amelia. Heroine. There's a fine line between willfully stupid and TSTL. She treated it like a balance beam. Her brother (unsure if older or younger) ran away from home so she took off after him...to Deadwood. This is the middle of the 19th century. She is neither an orphan, nor a widow. She got one of her father's employees to go with her, but I'm not sure how that's supposed to protect her against indian or gang attacks. Further, her clothing from the description likely needed a lady's maid to assist. Seems awfully modern in her thinking, particularly with regards to her virtue.
Parker. Brother. He's 22 going on 12. His first reaction at seeing his sister was delight, followed by getting pissy because she disapproved of him being in a brothel. His first reaction SHOULD have been horror, as in "what the $#@! are you doing here sis? This is no place for a lady!" and I don't mean the brothel; I mean the town. That pretty much set the tone of his presence in the book. He'd get pissy about something, ride into town, get plastered, gamble with money he didn't have, and visit the brothel where a girl he fancied worked. It was one of these excursions where the saloon owner came to pay him a visit, only to run into sis and proposition her to pay off the brat's debts. Boy wonder was more ticked off that his partner kissed his sister than concerned that his idiocy was endangering her.
Gabe. Hero. He's a former accountant turned gambler after his pregnant wife ran away with some dude with shiny and faked her death. He has no intention of offering for the heroine, but still pursues her. That's ok because she doesn't care about a little ol' thing like virtue (but her future husband might). He's in the process of pushing her away when his wife shows up from the grave.
Morgan. body guard of sorts. Mostly useless. Also an early proponent of gun control evidently. He's a big dude but is afraid to fight.
Mattie. Madame with a heart of gold
Big Jim Driscoll. Bad guy. Not implicitly said but likely the dude with shiny who lured Gabe's wife away. Owns local saloon, shantytown, and plays mob boss.
Samantha. wife. She's spent the past 10 years being a ho. Informs Gabe that he's a father. Tells him if he divorces her, he'll never find the kid.
And a few other minor characters.