Who is Spencer Hastings from Heartless and what is their importance?

Spencer Hastings is one of Ali's best friends and Ali's neighbor. As Spencer starts to receive text messages from "A," the messages lead to Spencer finding out Ali's mom and Spencer's dad have been having a long-term affair. In fact, Spencer finds out that Ali and Jason are likely her half-siblings.

More: http://www.bookrags.com/studyguide-heartless/characters.html