Kelsey Pierce was Spencer's classmate at Penn over the summer. It's revealed in the prologue of "Ruthless" that she and Spencer used drugs and were arrested. Spencer asks Hanna to help her frame Kelsey for using drugs, and Kelsey is arrested, unknowingly framed. The next time Spencer sees Kelsey after framing her is when Amelia invites the St. Agnes Charity Chamber Music Group over to their house. Spencer then has a flashback of Phineas, a boy Kelsey and Spencer meet at a bar, giving them the drugs Kelsey is later arrested for. Emily also has a past with Kelsey, but doesn't immediately recognize her name, since Kelsey introduced herself as Kay.
Emily seemed to have a crush on Kelsey, describing her as "a sexier, green-haired version of Ariel from The Little Mermaid" the first time they met, as Kelsey, alias Kay, was wearing a mermaid costume then. Kelsey may be bisexual because at the party where she meets Emily she kisses her. At the end of Ruthless, Kelsey, depressed, overdoses on Easy A pills and attempts suicide by jumping into the quarry where Mona died, but Emily saves her and Kelsey is sent to the Preserve to be in rehab.
In Deadly, it's mentioned that she's still a patient at The Preserve.
Kelsey is described as having "cinammon-red hair," "sparkling green eyes," and "a smile on her face that says I have a secret and I dare you to get it out of me".