For a moment there i forgot about Asterion's brother and thought you had just dropped a major plot twist.
As for MC's pattern, well i've mentioned somewhere in the forum before how these self insert protagonists feel like non-characters to me, yours doesn't even have a default name, i had to come up with some of my own. For example the leader background MC i called Leander, for math, Matheus, that kind of thing. Anyway, what are the few things we do know about the MC that doesn't make him an average person? The deed to the hotel, making him the master of a pocket reality where he can create many things out of thin air. He is also very much empathetic, and is generous with kindness to others. unless he sends asterion to the valley once, He is also rather chill, confortable with being behind the scenes. These qualities are actually fairly similar to the strength arcana from tarot, not just the external strength from having unrestrained powers in this realm, but also the inner strength from being a balanced individual who is secure of themselves in that they do not abuse of their powers to assert dominance over others. this arcana is often represented by a girl and a lion, the symbolism being that this girl is able to put her hand inside the lion's mouth and they won't bite her (kinda like that contract with the armband). Now, how to translate this into a banner? It would have to be something that is strong and safe. this is where i can't help too much anymore, but my one suggestion would be a shield inspired design for the banner, although i'm unsure a shield would fit well with the themes of the story.
Edit: Wow, i kinda spiraled here. maybe just do the armband, as not only it evokes the same imagery from the strength arcana, it is also the physical manifestation of MC's promise to never hurt Asterion by sending him to the valley.