He is a soldier serving under Ser Gregor Clegane who specializes in interrogation and torture techniques.
After Arya Stark, Gendry and Hot Pie are imprisoned in a village near Harrenhal taken over by the Lannister army, they hear the Tickler interrogating a prisoner. The following day, another villager is selected by Gregor Clegane. The Tickler has the prisoner strapped and a bucket with a rat inside put over his chest. He interrogates the prisoner about hidden gold, jewels and the whereabouts of "The Brotherhood" and which villagers are helping them. When the prisoner denies knowledge, the Tickler has Weasel heat the bucket with fire to make the rat claw its way through the prisoner's chest. After a short time, the prisoner denounces a butcher named Gaines and his son, but the Tickler continues with the torture, killing the prisoner.
The next day, it is Gendry who is chosen and the Tickler goes through the same process again, but Gendry is saved when Tywin Lannister arrives and decides it's stupid to kill able-bodied workers.
Jaqen H'ghar wants to repay Arya for saving his life, when he was trapped in the burning wagon. He offers Arya Stark three deaths as balance for his life and the lives of Rorge and Biter. She robbed the Many-Faced God of three deaths and must name three to replace them. The torturer is the first man she names (she doesn't know his real name, just that they call him the Tickler). Arya is talking to Gendry when they hear a scream. They run out to see the Tickler dead on the ground, his neck broken. Above him on the walkway, Arya sees Jaqen H'gher. He places one finger up next to his cheek.
The chief interrogator at Harrenhal, the Tickler would put a starving rat in a bucket and bind it to his victims. Arya Stark named him as one of the three kills that Jaqen H'ghar owed her.