Hello @CosmiKing , I am honored to be the one you have come to for advice, and what I'd say is GO FOR IT MAN! I fully agree that Marshadow deserves some love.
Now about how to make the little guy not OP as all heck. Thankfully your not the person who gave their Mc a mythical as a first partner, so taking inspiration from that.
Try giving Marshadow something that's holding him back, not just emotionally but maybe even physically if you want. Maybe from a battle from the past or some evil group.
One last thing, I want you to remember that this will be YOUR story. So do what you feel is best, and don't give up my friend!
Kamen out ;-)