Pitezel's a carpenter who joined Holmes' construction of his hotel in November 1889. He's one of the few guys that Holmes actually trusts, so he keeps him employed. Pitezel is married to Carrie Canning of Galva, Illinois, and he's the father of five children. Pitezel does all sorts of things for Holmes, but the extent of what he knew about Holmes' dark habits are unclear.
A district attorney would later say:
"Pitezel was [Holmes'] tool […] his creature." (1.5.30)
Pitezel becomes a household name in the national search for his three missing children. What happens is this: after the fair, it's time to leave. Holmes takes out a life insurance policy on Pitezel. He kills the man, uses his children to identify the body, but then gets caught on insurance fraud.
Pitezel is Holmes' one mistake with his victims. He becomes too close to the man, allowing him to enter his personal realm and learn some of his secrets, and it's this relationship with Pitezel that leads to Holmes' demise.