Kairi's Letter

In Kingdom Hearts II, Sora and Riku are stranded in the Dark Meridian and receive a letter in a bottle from Kairi. Riku takes it out of the bottle, looks over it quickly, and says to Sora, "I think it's for you." Sora then reads it aloud.

"Thinking of you, wherever you are.  We pray for our sorrows to end, and hope for our hearts to blend.

Now I will step forward to realize this wish.

And who knows:

Starting a new journey may not be so hard,

Or maybe it has already begun.

There are many worlds, but they share the same sky.

One sky,

One destiny."

After this, the "Door to Light" appears before them. Sora stands up and turns to Riku, who has an injured leg. "We'll go together," he says, and holds out a hand. "Yeah," Riku replies, and gets up. All of a sudden, they fall into the water at Destiny Islands.

Trivia

 * Strangely enough, if one waits at the main menu screen of Kingdom Hearts long enough, a small cut scene starts playing in which this poem is shown in with "Hikari"'s instrumental orchestral version playing as background music. The same poem is shown when one waits long enough at the main menu screen of Kingdom Hearts II, only this time, an orchestrated version of "Passion" is used as the background music. This shows some level of continuity planning for the sequel on Square's part. The letter also appears at the front of the game manual for Kingdom Hearts, again showing that maybe the letter is a sort of slogan verse for the series.
 * It is possible that the letter got to the Dark Meridian the same way Sora and Riku got back to Destiny Islands, via the Door to Light, only the letter went to the Door to Darkness.