Mickey Mouse appears as King Mickey in the Kingdom Hearts series. While he's still the same Mickey we know and love, just like Donald and Goofy, he has some character development in the games that is unique to that version of the character.
But all of it is still true to who he is. Mickey appears in the three main installments and in many of the spin-offs. Here's what we learned about Mickey.
He Is Royalty
The first big thing we find out about Mickey in the Kingdom Hearts series is that he's royalty. He's King at Disney Castle, and Minnie is Queen. We don't see him much in the first game, however. Near the beginning of the story, Donald goes in to see Mickey and finds that he isn't in the throne room. Instead, he finds Pluto with a note from Mickey.
Mickey's note informs Donald that there's "trouble brewing" and he's set out to do something about it. He also sends them off to Traverse Town, where they meet Sora. We see more of Mickey doing his kingly duties in other games.
He Is Selfless
In the first game, Mickey shows up to help fend off the Heartless as Sora, Donald, and Goofy close the door on the darkness. He and Riku are being closed off on the inside, but he's ready to do what he needs to do. In A Fragmentary Passage, we see Mickey come to the aid of Aqua while she's in the Realm of Darkness, leading to the moment where Mickey helps Sora and the others close the door.
Also, in Birth by Sleep, we see that Mickey was like this even when he was training. He comes to the aid of Ventus when he's fighting Vanitas. At any point in his journey, Mickey is ready to put himself before others and risk his life to save them. As he says in Kingdom Hearts II, "Helping others should always come before asking others for help."
He Is Optimistic
In Chain of Memories, Riku tells Mickey that he fears that darkness will consume him. He begins to ask Mickey to destroy him if it comes to that, and before he can finish, Mickey responds, "Of course! I'll be right there to save ya!" When Riku tries to push the matter, Mickey says that no matter what, he'll be there to help.
When we see more of Mickey in Kingdom Hearts II, he's certainly a boost to Sora. This is especially helpful in that story, with Organization XIII against them.
He Is Curious
While Mickey would certainly never let curiosity put others in danger, we see his curious side in Chain of Memories. When he sees how Riku fights with both the light and the dark, he wants to find out where that road leads. He then decides to set out with Riku. This is also when Riku finally stops calling him "Your Majesty" and starts to call him "Mickey."
In 358/2 Days, Mickey meets up with Riku and discusses how they've been tracking Organization XIII. It's more than just the good guys trying to stop the bad guys because Mickey is putting that curiosity to work. And they certainly couldn't have done it without Riku, who Mickey saw the potential in even when Riku couldn't see it himself.
He Is Loyal
Mickey makes a promise to Riku to not tell Sora about what happened to him while he was in the darkness. He even gets pressed on it, but he can't allow himself to break his promise. We see again and again that Mickey is very trustworthy, and nothing will make him break that trust. Mickey also promises Riku in 358/2 Days that he will look out for their friends.
Mickey spreads this kind of feeling to those around him as well. He tells Aqua in A Fragmentary Passage that the old Keyblade wielder saying, "Let our hearts be our guiding key," should be used again. He decides he'll use the words himself, and he inspires others with them.
He Once Existed In Black And White
In a fun twist on the usual worlds of the games, Sora and the others find themselves in the Timeless River in Kingdom Hearts II. Not only is it a recreation of old cartoons, but it's in black and white. When we see Mickey, he's in his 1920s design and doesn't speak. It's also implied that this is actually in their past.
This world gives Kingdom Hearts a vibe like it's part of a bigger continuity, that maybe Mickey did once have simple adventures facing Pete before he became King at Disney Castle.
He Can Be Secretive, Even By Accident
Mickey often knows more than anyone else, and he doesn't always tell them. It isn't that Mickey wants to keep it from them, it's that he has a lot he has to do in his own journey. When they first see him in Kingdom Hearts II, he takes no time in telling them they must board the train. When they turn back, he's already running back into Twilight Town.
When Sora and the others find a picture, and Mickey tells them the picture is of Ansem, they show him a portrait of the villain from before and try to remind him that he was Ansem. Mickey responds, "Oh, that's right, I never finished explaining." He then tells them how that was the Heartless of Xehanort, who just pretended to be Ansem.
He Is A Warrior
Mickey is one of the best fighters in the whole series. He shows his skill with a Keyblade often, whether in a group or on his own. When the group first sees him in Kingdom Hearts II, he slashes through a group of Dusks like it's nothing.
As seen in the Country of the Musketeers in Dream Drop Distance, he's worthy of being one of the Musketeers, and so are his close friends. It's true that Sora, Donald, Goofy, Riku, and the rest are great fighters. But Mickey more often than not makes it look effortless.
He Is Brave
Mickey's bravery shines through many times. The first time we see him in the first game is when he helps seal the door, and it sets the tone for who he is in the series. At that point, he'd lost track of Aqua, and had no real means of getting out of the Realm of Darkness. It wasn't until he found one of the cards seen in Chain of Memories that he gets back to the Realm of Light.
In Kingdom Hearts II, when Goofy gets hurt, Mickey is ready to charge into battle against everyone. Mickey's sense of honor and bravery seems to have no limits.
He Trusts His Friends
By the end of Kingdom Hearts III, everyone has come together. Mickey, Sora, Donald, Goofy, Riku, Kairi, Axel, Terra, Aqua, Ventus, Roxas, and Xion. Mickey is perhaps the wisest of them all, and he's helped many of them along the way. But he doesn't go it alone. Even though the seven Keyblade wielders needed to come together, Mickey truly believes in every single one of the people who fight by his side.
Of course, the heroes are able to triumph over Master Xehanort. This likely won't be the last we see of our heroes in this series, but whatever happens, Mickey will have his friends, and they'll have him to look up to as well.