We only meet Sam's stepfather, Haden, through flashbacks because he's dead when the story starts, but he still makes a pretty big impression as the only stable male figure in Sam's life. In all things, Haden is pretty much the anti-Kevin. For one thing, he accepts Tia for who she is, witchiness and all: "Haden never made me feel ashamed of anything," Tia says. "He found out what I was and didn't care. In fact, he seemed delighted" (14.14). So he's open minded and a lover, not a hater.
Maybe this is because Haden's a little cooky himself. After all, he did request the Rolling Stones anthem "You Can't Always Get What You Want" as the theme song for his funeral. Still, the guy was also one heck of a dad—so much so that Sam thought of him, not Kevin, as his real father. Check out the story he reminisces about while waiting for Douglas at the zoo, where Haden "still bought [Sam] ice cream" after he chose to donate his allowance to the zoo's fundraiser for new facilities—and donated twenty bucks himself (7.15). Aw.
But why have Sam's dead stepfather in the story at all? Probably because his death is yet another way Sam's been dealt kind of a raw deal. It's bad enough that his real dad abandoned him and his mom, but getting a great dad and then losing him provides yet another clue to Sam's lack of direction.