Maki Aika is, much like Yurika, a magical girl however she is part of "Rainbow Darkness" who are are extremist organisation as of now their motives are unknown. She is Yurika's rival/arch-nemesis and she is the antagonist for the fourth arc of the anime and the volume 5 of the light novels.
Having been betrayed by everyone in her life when she was growing up, Maki became a cold, distrusting fighter. As Dark Navy, Maki was ruthless, caution and was fine using magic for selfish reason without regard for whoever she hurts. Despite her calculate thinking, Maki made a lot of misunderstanding when she first appeared. Such as believing that Yurika "convinced" everyone that she was just a cosplayer as part of her "plan".
However, compare to the other members of Darkness Rainbow, Maki is actually quite honest; hating lies and betrayal (although she knew not to tell normal people that she was a magical girl). She also hated makeup and fashion since she saw them as lies meant to make her look better than she was.
Maki personality and life started turning for better after she was saved by Koutaro Satomi during the ski trip in Volume 8. She became more cheerful and friendlier to everyone well letting go of her Dark Navy personality. She also started becoming becoming more feminine, putting more thought into how she look and dressed despite her previous hate for makeup and fashion.
Maki seem to have a capitalist side to herself. She likes to charge people for small thing they would want (Ex. true love chocolate on Valentine Day for boys who pay her). Shizuka Kasagi believe the reason for this is because of her life, Maki wants a easy to understand bond between her and other people.