Reili
the corrupted core will probably come from the brother huh. makes the most sense. oh i just remembered, after shiro class ups won't she be a C rank monster? i vaguely remember that being someone else's class up requirement. her secret will probably be blown after this quest. well it's probably about time she told the others anyway. i mean if lisa does come along (and i do hope she does) she'll already know about shiro's nanomancer class so there really wouldn't be a reason to keep it a secret. if she was worried about the others maybe one day turning on her because she was too strong than she really shouldn't worry, because she's already stronger than most in her tier anyway. they'd probably just think something along the lines of "well that's just shiro for ya". as far as being a monster goes, the only thing she'll really have to worry about is making sure lyrica doesn't decide to change class to monster tamer so she can keep her all to herself.
If anything I think lisa brother turned into A demon Cuz he got The info what The Light goddess plans for his sister I mean If it were to be Choose the demon side and get rid of the goddess so his sister will live or Help The Good side but lose his sister If he is a siscon Which i think he is The choice is obvious