You meant DeeDee, right?
Corey uses Sybrina to establish that Martin was screaming for help, therefore Zimmerman was not acting in self defense. Obviously, this also goes to the charge of depraved mind murder.
DeeDee's testimony is part of the murder charge as well, as she is the only witness who creates Zimmerman "stalking" and persistently working to close distance with Martin, while Martin is scared. But DeeDee's testimony stops short of the fight, so she can't participate in the self defense argument.