In Kingdom Hearts I, it is an ability:
Scan
AP: 1
Location: learned at level 15 (Sword), 12 (Shield), 9 (Staff)
Description: View an enemy's HP gauge during battle.
It is also an ability in Kingdom Hearts II:
Scan
AP: 2
Location: learned at level 4 (Sword), 4 (Shield), 4 (Staff)
Description: View an enemy's HP guge during battle.
Simple, right?
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Enemies may drop HP orbs, MP orbs, Munny, or synthesis materials. Some enemies are even known to drop weapons, shields and accessories.
I don't know the number, but it reaches the end of your MP bar
He has 3000 HP in both Kingdom Hearts games, and is considered one of the hardest bosses in the whole games though, luckily, is also optional in both games.
The One-Winged Angel is obtained after you defeat Sephiroth, a Final Fantasy character who is arguably the hardest foe in the game, at the Platinum Match at Olympus Coliseum. However, this only occurs in Kingdom Hearts: Final Mix, which is only available in Japanese; other than experience, there's no reward for defeating Sephiroth in the regular Kingdom Hearts.
When you complete certain parts in the game your hp bar will increase. If you want to increase it more, you can meld fire and sliding dash and attach a soothing crystal (if it isn'e a soothing crystal, sorry) and then you have fire dash with the ability hp boost. You can repeat this melding two more times if you want more hp.