Hi there,
spoiler also~!
Yes rejecting Jeth will make him upset with you and after he gives you a warning (scar) if you don't go along with him romantically the next opportunity, he will refuse to continue bonding. ( he is indeed all the crossed out things. XD;;) it is possible to lead on both mikaylis and Jeth simultaneously though up until accepting their eternal pendants (bond 6). Then whoever you accept first will be locked in their route and the other will try to give their pendant but see you have their rival's and be super salty/jealous. Thanks for trying out the game and I hope you enjoy~!