It's kind of hard to explain without the feel of the scenario itself, but i'll try to make it short
Note that both the the "this world version" heroine (e.g. Ayaka) have similar reasons with the "isekai version" heroine (e.g. Arnett), but "this world" heroine have an extra reason (always a special feeling for the human MC XD)
Red
Arnett = She's too aware of what people think about her, she's afraid of rejection. Maou accept her as she is as "his woman"
Ayaka = Arnett Reasons + She loves her childhood friend (which of course is the MC)
Purple
Aquila = She fights because the goddesses told her to do so. She always did as she was told. MC asked her what she wanted to do,to not only follow what others want her to do. After a lot of thought, she said that she wants to defeat MC. After another defeat, MC told her that she's wrong. It's not what she wanted to do, in fact at the moment she doesn't want to do anything. MC told her to rest until she finds what to do next.
Natsuki = Quite the same, as an heir to a big company her whole life she only did as she was told. She's unsure about her future, until she is "corrupted". And obviously, since it's a galge, she also loved MC.
Black
Labyrinth = Already corrupted from the beginning, corrupted even more by the MC. She WANTED the MC to be her and planned to lock him up and make him hers. MC established dominance LOL XD
Kokoro = The only heroine with different reason to fight, unlike her isekai counterpart. She accepted to fight MC because she's a chuunibyou. Doesn't really care that much about defeating MC. She even said that if she knew Kazunori is also Maou, she would gladly sever her link with labyrinth. She's afraid to be "normal", and although she could pretend to be like the other, she wanted to be accepted as what she is, a chuunibyou. She loves MC, because he accepted her the way she is.
Yellow
Citron = (Not sure if i romanized her name correctly). She fought for people's smile. MC said that he is smiling as he screw her. She said that if MC smile, other people will be sad. So genius MC brought other people to gang her, told her that they're smiling too. Then MC pointed out that while she made other people smile, she ignored her own smile, and her smile she put on all these time is a made up lie. Maou told her to care about her own smile (including being his woman and enjoy sex with MC ofc).
Reon = Ex-student council president. But the student council members always leaned on her. She kept being relied on and she never refused their requests. Deep inside she long to have someone to rely on. And of course it's the MC.
Blue
Silence = Small boobs complex. She can't speak out what she thinks and afraid of other people. MC told her to believe in him and be his.
Nao = Similar reason with Silence. Also she can't speak out that she actualy liked MC.
White
MuniMuniFein = Childhood friend with MC, actually liked MC for a long time, but her feelings conflicted with the fact that she's a goddess and a goddess must defeat maou. MC made her realize her desire to be with MC.
Chihiro = Clumsy teacher, MC supported her, so she became infatuated with MC.
Sorry for the long post and if there is anything that you want to ask, ask away. I'm a bit tired since i have yet to eat my dinner to type this (since i'm really hyped right now), so i might have typed some weird unintelligible words. XD