Nicole is the mayor's daughter. Nicole is described to have a pixie face, blue eyes, blond hair, and she's about 5'1". Nicole is on the swim team and in choir. She was closer to Serena than Maya, but Nicole and Maya became closer in order to comfort each other after Serena's death. They do not have much in common, other than they both lost Serena.
In The Calling, it's revealed by Sam that Nicole is mentally unstable. She was the one who roofied Maya at her party the night she kissed Rafe, and she was the one who drowned Serena because she wanted Daniel. At first Maya doesn't believe this, but when she finds Nicole, captive by the Cabals, and is reluctant to rescue her, Nicole starts ranting at her, hurling insults, and intentionally gives away her position to the guards, nearly getting Maya killed. This is enough to convince Maya of Sam's claims.
In The Rising, Nicole's true mental state is revealed by Sam to everyone in front of Nicole when Maya meets her at the Nasts facility. While Nicole denies this, Sam antagonizes her by insinuating that Maya and Daniel slept together, causing Nicole to become outraged and attack Maya on the spot. In her furious rants, she confesses to having killed Serena. She is revealed to be a xana, which is how she killed her, by being able to hold her breath long enough to drown her, as she could always hold her breath longer than Serena could. Later, Nicole is drugged by Dr. Inglis and released in Maya's room, where she attempts to smother her with a pillow, nearly killing her, however, Maya is able to fight back, and ends up nearly killing Nicole. She leaves the unconscious girl in the room as she tries to escape. Nicole is later seen, bound and blindfolded, being taken into a van during the evacuation of the facility. It is later confirmed that she is being treated by the Nasts, but she will never be let near Maya or her friends ever again.
From The Gathering:
"I glanced over to see Nicole Tillson, the mayor’s daughter, at her locker. She looked from me to Rafe, concern darkening her blue eyes. I mouthed "Save me," and her pixie face lit up in a grin.
She scampered over. "Oh, thank god I found you. Did you read that chapter for history? I was halfway through when Hayley called and I never got back—" Her eyes widened as if she'd just noticed Rafe there. "Oh, hey, Rafe. Do you mind if I steal Maya's brain for a minute? I seriously need it."
We stopped at her locker so she could get the book she'd come for. I had to help her with that. I'm only five foot five, but Nicole's at least four inches shorter and the guys like to stick her books up where she can't reach them.
Nicole was on the swim team and in the choir, so she'd been more Serena's friend than mine. That changed once Serena was gone. We'd kind of taken on each other as replacement pals. It wasn't a great fit—we didn't really have that much in common—but it filled a gap."