in KH2 final mix+ yes but in regular KH2 English the closest thing to a secret boss is Sephiroth.
Xemnes
Demyx of Kingdom Hearts 2 height is unknown. Demyx has a small role in Kingdom Hearts 2 and he is also one of the boss fights.
is that the dragon?
Im afraid that that's only on Kingdom Hearts 2 final mix.
There is no possible way of finding Xemnas in kingdom hearts. You are only able of finding him in kingdom hearts 2, which he is the last boss in the game.
The last boss is Riku.
Xemnas in the world that never was
I think when you die at a boss battle you can be Mickey
They are in Agrabah and Neverland (Clocktower)
Not that I know of
I think Kingdom Hearts 2 is better, because you will probably understand what the storyline is, you get forms which allow you to become a bit stronger, enjoy the characters in every world and if you die in any boss battles you may play as Mickey and and choose to hurt the boss or revive Sora!
Terra is unlocked as a secret boss after defeating the final boss fight, Xemnas, and saving your game since you're now able to save after the battle. Terra appears at Disney Castle's Hall of the Cornerstone.