Scout has seen and heard about Dolphus Raymond before, but she doesn't get to meet him until she escorts Dill from the courtroom during Tom Robinson's cross-examination. "Dill had started crying and couldn't stop," so Scout led him outside until he could regain his composure. She tried to explain to Dill that it was Mr. Gilmer's job to "get good'n down" on the defendant, but the prosecutor's treatment of Tom, including repeatedly calling him "boy," made Dill sick to his stomach. Outside the courthouse, Dolphus Raymond had just the thing to make Dill feel better. He offered Dill a drink from his bottle hidden in the sack, but Scout was wary of the offer.
As Mr. Dolphus Raymond was an evil man I accepted his invitation reluctantly... Somehow I didn't think Atticus would like it if we became friendly with Mr. Raymond, and I knew Aunt Alexandra wouldn't.
Despite Scout's warning, Dill "sucked on the straws, smiled, and pulled at length." It was just Coca-Cola--"plain Coca-Cola."
The quotation can be found in the second paragraphy of Chapter 20.