who do scout and jem refer as "pocket merlin''?

Scout and Jem refer to Dill as a "pocket Merlin". Scout says that when Dill joined them in their plays, he received the roles that she used to have to play, and they were always the "character" roles, roles that required a little more creativity. "Thus we came to know Dill as a pocket Merlin, whose head teemed with eccentric plans, strange longings, and quaint fancies."

Merlin is an allusion to the Wizard from the legends of King Arthur. The eccentric plans may also refer to the plan to get Boo Radley to come out of his house. The fact that they call him "pocket" Merlin, meaning miniature Merlin, may be because Scout says that, even though she is younger, she towers over him, so he is probably short for his age.