Charlotte Weis-Rex was one of the many children of a cult that surrounded bringing about the fabled "Horseman of the apocalypse", their goal was to wipe away what they saw was impure (which was basically a good chunk of the planet) and rule over the "unworthy".

Years had gone by, like many other the other kids she was kept away from "war", as to keep him focused on the task of ending the ruined world. When she was 13 she accidentally met him, and found he was just some doofy kid who can summon very powerful flames and heal from nearly anything. Their friendship grew over the years as they met in secret. Upon their 18th birthday, both her and "war" ran away from the cult.

It was an uphill battle, but the two manage to form their own lives in an Engelaland town far away.

To Nakamura she was the nicest person with a massive heart of gold, a real angel.

Charlotte,in reality is not a good person. Far from it, she wanted to control and bend him to her whims, to be the one alone to rule the world. The no magic rule was to prevent the cult from finding them, as she knew that would be her death. Besides that she was just an all around jerk to her neighbors, cashiers, orphans, pan handlers, charity bell ringing Santas.

Did she ever love Nakamura? Of course she did, but it's hard to tell if it was for his potential as a world destroyer or for his personality.