I have a theory regarding shade. It's a bit out there, but I believe he is Giratina in Disguise. Even though Giratina has only ever been mentioned once in the whole game in Coral Ward, the two do share heavy similarities. Shade's actions and way of speaking are that of a nigh-omniscient entity, he has powers connected to shadows, he's a shadowy figure, and his eyes and mouth remind me of Giratina.
There's also the fact that he barely intervenes, and when he does, his actions leave a big impact on the story. He saves you from the train crash, he directly progresses the plot after you beat him, he warns you about making the right decision in the waterworks, and he brings you out of the black hole. All of those actions coincide with Giratina's official canon role in the world, that being to correct any mistake that would, is, or has happened.
The fact Giratina is mentioned at all in Reborn, since they have no official prominent role in the story, is also something to consider. No other character shares similarities with Giratina. Shade is also the ghost-type gym leader, and Giratina is dragon/ghost.
There's also Shade's story. It could be that Shade deliberately humiliated that trainer so as to teach them a lesson, only losing to them on purpose when they changed, further supported by how easy shade's battle actually is. As to why he was so interested in a no-account trainer that was only mentioned once, and the ramifications that would happen if Shade did not stop them, that is beyond me.
There's not much else I can think of, but you guys are more than welcome to add your own thoughts. Also, this is my first theory so feedback is appreciated.
Lastly, shout out to uberle for making his/her own theory on shade: