I had a crazy idea and am just putting this here before I forget it. There could be a legend (I don't know who) in the later gens like 8,9 or 10 that would be a fanatical fan of the MC similar to the human fans of Meloetta. It would start with that the MC needs to have collected at least half of the total number of legends up to that region (or all of them) so that his fame would be great enough to catch her attention otherwise any less and Maple would no be able to find any trace of the legend since she would not want to be found. She would be such a fanatical fan that she learned human language and some of the tech to search for info like how the human fans of Meloetta learned her language. Don't underestimate a fanatical fan, give them enough time they can adapt to be able to know more about the MC than the MC knows about themselves.
So when the MC arrives at the region she is at she absolutely wants to join him but got it in her head that there can be fake MC's around so made a test that only the real MC can pass with the journey and battle or quiz. A question could be "who was the first "legendary" the MC caught of the first region" a little trick question since not asking about subs or mythical legends so Mewtwo. In the journey part the MC could find some shorts on the ground but has a familiar feeling that he has seen them some where before but keeps going. Once he reaches and beats the legend he sees a lot of shorts almost made into a blanket and realizes they are HIS shorts that he left in his house when his journey first began. The legend had somehow traveled all the way to his house and collected them so she can smell him at any time. She would be ecstatic to finally call him master and could literally jump into the masterball out of excitement. She would already know about all the legends he has caught so far and even know the layout of the ship before being on it. She would also secretly love watching the MC making love to the other girls but most of all loves it when he makes love to her. She would be just that kind of a fan that being closer to him and having his kids and seeing him work make her happy and maybe she could keep track of how many kids the MC has with the legends and humans similar to the list someone had made with the min and max number.