In Final Fantasy XIII, what is Fang & Vanille's relationship? Are they companions or just friends - fan art seems to suggest the former.
It would seem that they have been friends since childhood.
Fang and Vanille hail from the village of Oerba on Gran Pulse. During the War of Transgression between Pulse and Cocoon, Fang and Vanille lost their families at the hands of Cocoon's l'Cie and fal'Cie and befriended at Oerba's orphanage
Vanille of the Dia clan, and Fang of the Yun clan both lived in the communal village (everyone lived together in one residence) of Oerba which revered the fal'Cie Anima. They are technically best friends, but practically family as they see themselves more as sisters.
Explanations: I've played through the game several times, and its easy to see that they're definitely very close (I don't think anyone would argue that). As I said, technically "best friends". Another theory is whether they have a romantic relationship, I don't believe this to be the case because they never kiss or do anything more than what looks like a friendly hug (no hands rubbing all over each other or anything). There is a (probably famous) scene of them; just after finding each other from being separated for many days (since Euride on Day 5, I believe), Fang lifts up Vanille's skirt to check her brand which is RIGHT beneath her butt cheek, lol like I said very close.
Now, for me personally once I've been "best" friends with someone for quite a few years I really consider them more like family. In the case of Vanille and Fang they literally grew up together in a very small village and they probably even shared bunk beds (they show two sets of bunks in the house that everyone supposedly lived in) considering they did sleep in the same house. It's also very obvious that Fang takes the role of protecting Vanille very seriously, similar to how a big brother or sister would act. So, again, I guess it is just my theory that they would view themselves as "sisters," but when you actually grow up living with someone for many years its just natural to want to think of them as family, and it seems like Fang and Vanille's behaviors reflect that.