Marcia is a friend of Cherry Valance; the two sit with Ponyboy and Johnny at the Nightly Double. Marcia serves as a contrast to Cherry, accentuating traits that Cherry has through her (Marcia's) own lack of them. Ponyboy picks up on this fact after they have talked for awhile -
"I realized then that Marcia and Cherry weren't alike. Cherry had said she wouldn't drink Dally's Coke if she was starving, and she meant it. It was the principle of the thing. But Marcia saw no reason to throw away a perfectly good, free Coke".
Marcia is a nice girl who takes things as they come, never looking too far below the surface. She is easy-going, fun-loving, and accepts things for what they are. When Two-Bit explains the gang's nonchalant attitude towards fighting, she is "unconcerned", while Cherry, perceiving the danger and potential waste involved, takes it more seriously. Cherry, like Ponyboy and Johnny, is more perceptive than Marcia, and a deeper thinker. She will not humor Dally,who has treated her rudely, as a matter of principle, but she sees through his tough-guy facade, recognizing something within him that she can admire.
Marcia's character serves to illustrate that there are huge individual differences no matter what social class people belong to. Marcia is a good girl, but Cherry has that special quality that Johnny later calls "gold", while Marcia does not.