I hope people going all about cucking or NTRing are just joking. Non of the girls supposedly "stolen" from the Brave had any feelings towards him, he himself is just like your typical beta MC looking on his internal thoughts/monologues.
Princes was basically Heroes before she met Brave, same with Sucubi-chan, Fox-chan was supposed to join his party but it wasn't set in stone and she was "dude rescued me so I now love him for ever" so she just gone her love interest way.
If MC aka Hero manages to take away the pentako new girl after side-dude aka Brave manages to start a relationship with her with them having feelings for each other - then we could talk about NTRing/cucking. But if he does it without feelings between beta-rave and peta-chan then no NTR yet again
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At least if the whole cucking/NTRing isn't used as a meta-joke about party members not romantic/love stuff.