Anyone who doesn't want spoilers shouldn't read this. Seriously, don't do it.
Krista is the bastard heir of a noble family and had to either abandon her old name or be killed. Ymir's background we don't know completely, but we do know that she's a Titan and is worshiped by the normal Titans. They both were given a second chance at life, so Ymir says that they're kind of similar, but is pissed that Krista would just abandon her old name and become self-sacrificing, since she kept her name as revenge against those who oppose her and her new life, while Krista is doing nothing but making those who forced her out happy. So, when Ymir does the impossible to save someone and Krista asks her about it, they make a promise that, when Ymir reveals how she did it, Krista has to take back her name and live under it.
Later on in the manga, Ymir transforms to save everyone, but almost dies in the process. When she wakes up, Krista reveals her real name and, from that point on, everyone starts referring to her as that.
last edited at Jul 6, 2013 11:43AM