She is a close, maybe only, friend of Simon. Even though he has feelings for her, she does not have the same feelings for him. She, somehow, is able to contact Simon, though her side of the call is very choppy. He does not know this is her, though. Once he reaches the rooftops,  Sophie is there. She gets confused and/or shocked at Simons claim about there being monsters down there. He asks her if she remember how she even got here, but alas, she does not. She and Simon have a short conversation before she suggests sitting on the ledge of the rooftops. Simon takes advantage of the scenery, and confesses the fact that hes liked her for a while. After he confesses his love to her, she states that she doesnt share those feelings, but just wants to "get away from it all". Simon, of course, does not know what she means by this. That is when she jumps off of the rooftop, which leads into the Carcass boss fight. 

Sophies fate depends on the players choice, but no matter if you kill Carcass or not, she will always jump. In endings 1 and 2, in which you spare Carcass, she will get killed by S Simon. If you do not spare Carcass, and instead kill him,  Sophie will be spared and will live. If you play co-op, then no matter what, she will be alive.  In the cutscenes for E1 and E2, you can see cops walking in the bathroom, discovering Sophies corpse in the bathtub, covered in blood. If you do kill Carcass, it is a symbol of S Simon ending his guilt sorrow for Sophie, however if you dont, S Simons guilt will instead fester.