According to Bingo Morihashi, Red Queen and Blue Rose were created as a means to visually differentiate Nero's movement and gameplay from Dante.

"For the main weapon, motion artist [Yuichiro] Hiraki-san said he wanted a motorcycle-like sword. What does that even mean, though? He explained that the sword's handle would rumble and sputter like an engine, and everyone loved the idea. Using that sword was important for Nero's characterization, because it gave him this "biker gang" element. That plus his wildness signaled the shift to the current Nero."
Contributed by DrakeVagabond