When Jack first arrived on Sodor, he worked with the Pack to clear room for railway lines. Jack did well with Alfie - that is, until he tried to take some stone up a ledge but lost balance and tipped over. Jack learned his lesson, and redeemed himself by holding up a collapsing bridge until Thomas had passed over, at the cost of damaging his arms. Jack has since become part of the Pack.
He, along with the rest of the Pack, helped with the restoration of Great Waterton and worked with Alfie to construct a bandstand there. Jack also helped with the restoration of Ulfstead Castle and later saved Stephen and Thomas after the tracks they were on collapsed.
Jack was like a toddler at first, who was still learning about the world and how to stay out of trouble. He is brave and will not take any hassle, as seen when he stood up to Max when the latter was bullying him and Alfie.