In what way does Shotaro not have a backstory?
We know he grew up in Futo, loved the city and wanted to protect it, and so became Narumi Soukichi's apprentice. He was overly-eager and less capable until Soukichi's death, then tried to make up for his mistakes by taking up Soukichi's business and taking care of Philip. He is clearly the one who decides that Kamen Rider must be the name of a hero and also clearly names all of W's attacks.
From that we know Shotaro's motivation, we know what he was doing for a year before and after the series, and we know how his character transforms during this time. No, we don't know anything about Shotaro's family, but that's because the series emphasizes Shotaro's bond to the city and his supporting cast.
There is nothing we need to know about Shotaro that is omitted from W. Hell, most of what we know is outlined in the first two episodes! No, we don't know anything about his parents or potential siblings, but why should anyone care? Shotaro's family clearly had nothing to do with the story, nor would introducing that sort of thing improve the story in any way.
W amply characterized Shotaro by showing us how he acts and why he acts that way. Through that alone, Shotaro is already a far superior and more well-defined character than others who have more complicated (and often ridiculous, cliched, and unrelatable) backstories. A complicated character is not the same as a complex character, nor is either necessarily a good character.