In the novel, The Outsiders, who is Cindy (or Sandy)?I need an in-detail explanation of who Cindy is.

I don't believe there is a character by the name of
"Cindy" in The Outsiders, but you may be referring to Sandy, who is
Sodapop's girlfriend in the story. (There is also the mention of a Sylvia, who is a
former girlfriend of Dallas Winston.) Soda and Sandy often double-date with Steve Randle
and his girlfriend, Evie. Soda hopes to marry Sandy when she gets out of school.
However, Sandy eventually becomes pregnant--apparently not by Soda--and she is sent to
Florida by her parents to have the baby (and possibly be away from Soda and/or the
actual father). Soda nevertheless proposes to Sandy; however, the letters he writes to
her are returned to sender, leaving Soda heartbroken,