Generally speaking, the influence of individual "trendsetters" in fashion history tends to be much overstated, even in more modern times. Take my answer on Marie Antoinette here - there's a tendency to remember an historical person as creating and popularizing a style just because they're the only ones remembered today in conjunction with it, or it's assumed that because they were wealthy and influential in other ways they also regularly set the fashions, or that because they helped to popularize one style they also popularized others from the period.

For much of the Middle Ages, fashion as we understand it now - being up-to-date with changing trends - did not exist. The wealthy showed their status through the expense of their clothing: furs, rare dyes, imported fabrics. In the fourteenth century (the Late Middle Ages), we start to see fashion indicated with the fit of clothes - sometimes altering the shape of the body, especially the bust - dagging, full sleeves, etc. Expense was of course still important, but this is when status in dress became partly about knowing what's in fashion and following it, so this is about as early as you'd be able to find potential "trendsetters".