Alice is Edward's adoptive sister and the most supportive of his relationship with Bella because she knows how lonely he has been. When Bella sees Alice for the first time, she describes her as "pixielike, thin in the extreme, with small features. Her hair was deep black, cropped short and pointing in every direction" (1.110), and Bella often comments on how gracefully Alice moves.
Unlike the other members of the Cullen family, Alice remembers nothing of her human life – Carlisle and the Cullens found her when she had already been transformed into a vampire. We learn that Alice is unique in that she developed a "conscience" on her own, meaning that without any guidance, she decided against preying on humans. Later we learn from James that Alice was the only human he hunted who got away from him, and that an old vampire transformed her so she'd be safe. Then James killed the old vampire, leaving Alice alone to learn about life as a vampire. We also find out from James that Alice was locked up in an asylum as a human and given shock-treatments, since she had "visions" and her contemporary human beings thought she was insane.
Jump to the present, and Alice has a special power: she can see the future – but just what is being planned. In other words, the future can change. Knowing what is "on track" to happen, she can shift choices and actions to prevent that future, if it's undesirable. This occurs when James is planning to kill Bella – Alice sees James's plan, and helps prevent it.
Alice sees one possible future that is particularly disturbing to Edward: she sees Bella as a vampire. This is why, during her arguments with Edward over whether she should become a vampire, Bella says, "I'm betting on Alice" (24.287).
Alice takes a liking to Bella immediately and treats her like a sister. Not only does Alice take care of Bella while James is hunting her, but Alice also dresses Bella up for the prom. Though Bella isn't so happy about being Alice's "Guinea Pig Barbie," whenever Bella complained, Alice "reminded [Bella] that she didn't have any memories of being human, and asked [Bella] not to ruin her vicarious fun" (Epilogue.8). Overall, Alice is a warm, encouraging, and supportive presence.