My take on this story so far: The "S" girl gained Somi as a gopher quite awhile ago, and perhaps learned of Young's existence through her. Why did she use Somi as a gopher? No idea, except maybe she's just a manipulative bitch with time to kill. S gets some dirt on Somi and forces her to do increasingly bad stuff, to the point that Somi is ready call it quits, but she can't because she's afraid that S will reveal some personal stuff to the school. S then decides she needs a new gopher, but because she doesn't want to be implicated in any of the dastardly deeds she forced Somi to do, she decides to get rid of Somi, and make it seem like a suicide. S messages Somi with something like "all is forgiven, come to the rooftop, etc.". She then pushes Somi to her death. Now, she's forcing Young to do the same things she did with Somi, realizing that the two have similar, easily-manipulated personalities. There's strong symbolism indicated with the trees and strangling vines; the reference to the tree branch futilely attempting to escape the vines is a correlation between the current relationship between S and Somi at that time.
It's obvious that Somi confided quite a bit to S, and that information was used against her in the end. Both Somi and Young are stupid for allowing someone who is a blatant manipulator into their lives; S gave Young a backhanded threat right at the start, and Young should have spilled everything to Somi's mother and school right then and there. But, we wouldn't have a story if it were that simple, heh.