Christina is Tris's first friend really, which is pretty sad if you think about her spending sixteen years without one. Christina's a transfer from Candor, so she tells the truth all the time, which is a nice quality in a friend or an enemy. If she doesn't like you, she'll tell you straight up because, as she explains, "We try to be pretty honest about our feelings in Candor" (9.24).
But Christina doesn't wander around just dropping knowledge on people, which would get old really quickly. She's got more dimensions than being just a straight shooter. In fact, when Tris is all sore from a fight, Christina gets her breakfast and helps her get dressed. That selfless act prompts Tris to think that "Maybe there is some Abnegation in everyone" (11.11). Christina may be born Candor, and she may choose Dauntless, but Christina is a good friend who wants to help Tris through these problems. Hey, that sounds like a bit of Amity, too…
Christina is useful to Tris as a friend, but she's useful to us as—can you guess?—a foil for Tris. As a member of Abnegation, Tris never got to have a pet or makeup or choose her clothes—all things that Christina has some experience with. And Christina is willing to share her experience with Tris, giving her a classic 80s-movie style makeover (8.81-91).
Christina's may be way smart and way tough—just look at the scene where she dangles over the edge of the chasm to prove her bravery to Eric if you want proof of that (9.72-102)—but she's also a little more stereotypically girly than Tris, especially when she needs to talk about kissing Will. As she asks Tris, "Can you be a girl for a few seconds?" (28.179). Yeah, she's stereotyping what it means to be a girl, but it does point out a little difference between their interests and experience: Christina (makeup and guns) and Tris (guns and more guns and looking in a mirror four times a year).
But even though Christina and Tris had different upbringings, they share a lot of the same feelings and thoughts. Neither of them stayed in their original factions, and both of them have secrets they want to hold on to. When Christina says that "I try to be honest, but some things you just don't want people to know" (28.201), we know that Tris agrees since she just rattled off all of her terrible secrets, like the whole being Divergent thing.