It has to be Lilly, nobody else would get the same reaction from Clementine. If it was Kenny, she would be more relieved to see someone she knew. But if it's Lilly, Clem would think she was dead, and at the same time have that reaction.
Also looking at the mystery person's group, you can see that they are acting somewhat hostile towards Clementine's group, reaction that could easily be provoked by Lilly's leadership "skills". But if the mystery character is Kenny, the group would be acting more friendly.
I also suspect that Kenny or Christa would say something before Clem looks up, probably along the lines of "Clementine, is that you?" while Lilly is more likely to remain silent.