User blog comment:Avatar226/S-Class Fairy Tail Trivia Contest 2014/@comment-7577835-20131219235312/@comment-1776824-20131220000413

They have several personality characteristics in common; they share physical characteristics (red hair [initially long, but Knightwalker later cut hers] and a well-proportioned body). Their Magic both allows them to change forms for the purpose of battle: Erza's allows her to change her armor and switch out swords, whereas Knightwalker's allows her to change the form of her spear to one of several revealed forms (i.e. Explosion, Silfarion, Ravelt, etc.)...but if I'm not completely right then meh.