An HP UP increases the maximum number of HP (hitpoints) a Pokemon has. This allows it to absorb more damage from attacks before it faints.
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The Pokemon with the lowest HP is Shedinga. The base HP for this guy is 1, and stays that way no matter what unless you hack. It at least has the ability Wonder Guard to make up for this.
the two best ways would be to lvl. them up or use lots of HP ups.
You can buy HP Ups at the Shopping Mall 9 on Route 9.
Blissey has the most HP of all Pokemon.
There's no Pokemon with 1,000 HP. The closest to 1,000 HP is over 600-700 HP and that is Blissey.