Alice has become Bella's closest girlfriend and sister-to-be. She's an interesting character in that she combines utter girlishness with being a tough vampire with formidable fighting skills. She treats Bella like a human Barbie doll. She buys her clothes and designs her wedding dress. At the same time, Alice's partner Jasper calls Alice his "frightening little monster." No one worries about her in the fight against the vampire army, except for Bella.
Bella and Alice truly care about each other, yet Alice's loyalty to Edward supersedes her loyalty to Bella. She kidnaps her for sleepovers to prevent her from running off to Jacob and she keeps secrets from her, on Edward's request. Alice claims that it's because she knows better what Bella really wants. Case in point: She arranges a big wedding for her. "I'm trying to make you happy… You'll thank me for this someday" (21.74).
For more info on Alice, check out her "Character Analyses" in Twilight and New Moon.