Mickey's Letters

Mickey's Letter could refer to multiple different letters Mickey has written over the course of the Kingdom Hearts series. For example, the letter at the beginning of Kingdom Hearts, adressed to Donald, or the letter recieved in a bottle at the end of Kingdom Hearts II could be referred to as Mickey's Letter. Usually, his letter is sealed by a green Hidden Mickey symbol.

Kingdom Hearts Birth by Sleep~Reconnect. Kingdom Hearts
Sora is seen sitting on the Paopu Tree in the Destiny Islands looking at the sunset. Riku walks up to him from behind, asking if he has decided yet. Sora is holding the letter from the end of Kingdom Hearts II, saying that he has. Kairi appears, and Sora explains that there are still sad people, so he must leave the islands and put back everything that's been connected to him. Kairi smiles and places her Lucky Charm in his hand and requests him to hurry back.

Kingdom Hearts
In this case, Mickey's Letter refers to the letter addressed to Donald Duck. In it, he writes the reasons for his sudden disappearance, as well as instructions to Donald and Goofy to find the key-bearer.

Donald,

Sorry to rush off without sayin' goodbye, but there's big trouble brewin'.

''Not sure why, but the stars have been blinkin' out, one by one. And that means disaster can't be far behind. I hate to leave you all but I've gotta go check into it.''

''There's someone with a "Key"-- the Key to our survival. So I need you and Goofy to find him, and stick with him. Got it?''

''We need that key or we're doomed! So go to Traverse Town and find Leon. He'll point you in the right direction.''

''P.S. Would ya apologize to Minnie for me? Thanks, pal.''

Signed - Mickey

Kingdom Hearts (manga)
The letter reappears in the Manga series of Kingdom Hearts, with some slight differences.

Dear Donald,

Please forgive me for leaving so suddenly, but trouble is brewing, and there's no time to lose.

''I'm not sure why, but the stars have been blinking out... one by one.''

Hate to leave you, but I have to check into this.

As the king, I have a favor to ask you and Goofy...

There is someone out there with a Key''-- the Key to our survival. I need you and Goofy to find him and stick with him! Got it?''

Without that key, we're doomed!

So go to Traverse Town and find Leon...

Kingdom Hearts: Chain of Memories
The original letter to Donald makes an appearance in Kingdom Hearts: Chain of Memories, in the possession of Pluto.

Kingdom Hearts II
In Kingdom Hearts II, a new letter makes an appearance as a note in a bottle in the ending CGI cutscene.

Kingdom Hearts coded
In Kingdom Hearts coded at the end of the game it is revealed that the letter that Sora gets at the end of Kingdom Hearts II talks about how there is memories of another (Ventus) in his heart. It also states that Namine, Xion, Roxas, Axel, Ventus, Terra, and Aqua are connected to Sora's heart and in the future the memories must be awakened. It should be noted that Namine told King Mickey this in data Castle Oblivion and promised he would tell the real Sora, which he did.

I wanted to tell you immediately.

There are memories asleep inside you.

Those memory fragments connect to the future

Sora, Riku, Kairi

''The truth surrounding the Keyblade, passes through numerous connections, they’re inside your hearts.''

Sora

It all leads to you, everyone is waiting for you. The only one who can heal their sadness is you.

''Its possible that the travels up until now compared to the next trip may have been easy everything thought to be accidental was really connected the door to a new departure already seems to be opening.''

-Mickey