Story: believe it or not, Cinders and LS, were not friends at first. Basically Lightningstar and Cinders got into a fight..over something stupid I'm sure...Then when Cinders looked up, Ls reminded her of her brother (Which she accidentally killed, ya, ya, ya terrible backstory, etc.) and how they used to play. She deeply hurt her brother one time, and some part of her does not want the same to happen to Lightning. Cinders starts to see Lightning as her "Brother", thus dubbing Ls the nickname "brother" Here, LS isn't so fond of sparing others, but does so for Cinders anyways, obviously. Saying, "No, the water's sweet, but blood is thicker" would have been referred to as a warning, but finds LS locked in confusion whilst in deep thought.
@IcoQuest's turn!
