Sophie is an artificial humanoid created for the sole purpose of defeating Lambda—another artificial humanoid with incredible power who could basically be called her brother. Sophie is one of the strongest artificial beings that the scientists of Fodra ever produced. One way or another, she ended up on the planet Ephinea, memoryless save for her purpose: that she must defeat Lambda at all costs, including her own life.
Of course, before that she befriends Asbel and all of his friends. I don’t feel like relaying the entire plot of Tales of Graces, but what is important to know is this: Asbel officially and extremely legally adopts Sophie, despite being only a few years older than her. The whole party adopts each other as family, really, and Sophie learns what it’s like to live—simply live—and to love and be loved.
They come to an understanding with Lambda, rather than defeating him, which means he and Sophie both get to live. In Lambda’s case, he ends up sleeping in the back of Asbel’s soul—though he ends up being rather chatty for someone who is supposed to be asleep—and in my heart he and Sophie learn to be siblings as they deserve to be.
And, you know, maybe Sophie decides all she wants in life is to be Asbel’s daughter, and to have a nice garden. Is that such a crime? Certainly not.