In the series Edit
Wilf first appeared when he helped Todd and Viola cross the valley of creatures in The Knife Of Never Letting Go. He's described as very simple but warns them of rain hours before it actually happens. Later, when Todd is injured he lets him onto his wagon, on which he and his wife, Jane, are travelling to Haven along with the rest of his town. He says no one listens to him but when they do they believe him.
In The Ask and the Answer, Viola finds him after she has escaped from the House of Healing. He says he joined the Answer because they were going to take Jane so they could treat her after she fell ill. He didn't want to leave her so he joined and they used his wagon to ride in and out of New Prentisstown. Since he was so simple, the guards believed everything he said.