Dill earns Scout and Jem’s affection by being interesting and proving a third person to play their games.
Dill was different from other little boys. He looked and acted unusual. He was also a good playmate, taking part in Scout and Jem’s games acting out different book and movie plots. Since Dill had been to the movies, he provided new and interesting plots.
Scout realizes that Dill is going to be their friend when she tries to ask him what happened to her father and Jem stops her.
Dill blushed and Jem told me to hush, a sure sign that Dill had been studied and found acceptable. (ch 1)
Dill stays with them the rest of the summer, helping them try to get Boo Radley to come out. He is entertaining as an actor and idea man.
Dill is based on a real person named Truman Capote who was a childhood friend of Harper Lee. You can see her affection for the character in her descriptions of Dill.