After the Titan War, Clarisse got a pink streak dyed into her hair. It was hidden at the back, making it almost impossible to spot. Silena had always talked about how badly she wanted the two of them getting matching streaks, and Clarisse decided to do it in her memory.
No one had noticed, at least until she met Piper McLean. The daughter of Aphrodite had pointed it out immediately, and, because she reminded her so much of Silena, Clarisse told her the story.
After that, the two became friends, with a bond similar to that of Clarisse and Silena. The girls trained together, Clarisse trying to improve Piper's sword fighting skills before she left for Greece. They would have long talks, often leading into the middle of the night, in the woods where no one could see them. However, they'd never noticed the ghostly girl watching from the sidelines, smiling sadly at the two.