Translating Huillard-Breholles is definitely paying off. Although some of it so far is simply a list of what Piero did – i.e. he participated in such and such a negotiation, which could be gleaned from other history books, there is also detail from the Registrum that lists Piero’s admin responsibilities. It seems he was closely involved in supplying the army and castles, and chasing up money for the treasury. So he would have had his hands on quite a lot of money, thus not surprisingly attracting charges of embezzlement, but also generating possible resentment, for instance from those required to pay up monies, and from military commanders feeling the pinch.
I think the letters might give more about his actual personality, however, the difficulty I will have there is that they are in Latin, and Wiktionary is pretty poor on Latin!