Peshek was the miller of Rattay, Theresa's uncle, and a trainer in Lockpicking and Pickpocketing. He also serves as a trader, I mean "store-er", of stolen goods. He has at least one child, who is not seen.
During the Attack on Skalitz in 1403, most of Peshek's brother's family was killed. The only survivor was his niece Theresa, who narrowly escaped rape and murder at the hands of the Cumans, eventually finding her way to Rattay, bringing an unconscious Henry with her. Peshek, already annoyed at the influx of Skalitz refugees into his city, was less than pleased about footing the bill, not only to feed his niece, but also for the expensive care required to save Henry. But he paid nevertheless, and even agreed that Henry could live there until he got his own place.
When Henry had recovered, he assured Peshek that he would settle the debt as part of the quest The Good Thief. While the apothecary required coin, Peshek informed Henry he could work off his debt by paying him in coin as well, or Henry could do a small job for him. Namely, stealing a precious ring ... which is buried with a recently deceased convict.
Whatever Henry chooses, Peshek decides he is trustworthy, and agrees to assist with any stolen items that Henry might come across. Because buying and selling stolen goods is a crime, Peshek merely "stores" the items.
Peshek has a very cynical nature, having lived through wars and famines, convinced that nothing really ever changes or gets better. To survive, sometimes one must do away with morals. However, he does seem to care for his niece, taking her in without complaint and even caring for Henry at her request (albeit with more complaining). Henry also mentions in Courtship that Peshek will fret if Theresa stays out too late.