Heart Gauge

A Heart Gauge (Japanese: リライブゲージ  Gauge) is a meter for a Shadow Pokémon in Pokémon Colosseum and Pokémon XD: Gale of Darkness that measures how far it is from being able to be purified.

Heart Gauge progress
The Heart Gauge is split into five equal bars. When a Shadow Pokémon is first snagged, all five bars are full. In the same way that two Pokémon with a "full" HP bar may actually have different amounts of HP, so too may two different Shadow Pokémon with full Heart Gauges have a different absolute number underlying their Heart Gauges. The size of a given Shadow Pokémon's full Heart Gauge can be seen on the list of Shadow Pokémon. All other things equal, this means that a Shadow Pokémon with a bigger full Heart Gauge number will take longer to fully open its heart than a Shadow Pokémon with a smaller number will.

A Shadow Pokémon also starts out with its Nature hidden, only Shadow moves usable (one in Colosseum, two to four in XD), and unable to gain experience or friendship. As the Pokémon opens its heart to the player, the gauge decreases, and as the gauge passes certain milestones, some of the restrictions the Pokémon started with become undone.

A Shadow Pokémon always retains its Shadow moves until it undergoes the purification ceremony. So in Pokémon XD, where all Pokémon have at least two Shadow moves, dropping below one full bar on the Heart Gauge does not change a Shadow Pokémon's moveset at all.

Opening a Shadow Pokémon's heart
There are several ways in which to open a Pokémon's heart. The walking events run on a counter that is unique to each individual Shadow Pokémon. This means that, for these events, a Shadow Pokémon may open its heart at a different time than when all Pokémon gain friendship, and two Shadow Pokémon may open their hearts at different times.
 * Send the Pokémon into battle
 * Call the Pokémon out of Hyper Mode or Reverse Mode in battle
 * Walking, every 256 steps, while the Pokémon is in the player's party
 * Walking, every 256 steps, while the Pokémon is in the (Pokémon Colosseum only)
 * Use Scents
 * Place the Pokémon in the Purification Chamber (Pokémon XD only)

The event that occurred and the Shadow Pokémon's Nature combine to affect the amount by which a Pokémon's Heart Gauge decreases, as described in the sections below. These amounts also differ between Pokémon Colosseum and Pokémon XD. On average, the effects in XD are weaker than those in Colosseum; on the other hand, the average size of a full Heart Gauge for a Shadow Pokémon is also smaller in XD than in Colosseum.

In the case of Scents, the Scent used further affects the amount by which the Heart Gauge decreases. The Joy Scent multiplies the appropriate value below by 1, the Excite Scent multiplies it by 2, and the Vivid Scent multiplies it by 3.

In short, the actions with the potential for the strongest effects are: calling a Pokémon out of Hyper Mode or Reverse Mode, walking while a Pokémon is in the Day Care, and using a Vivid Scent. The absolute lowest effect of these is stronger than the absolute lowest effect of other actions, and their absolute strongest effect is stronger than that of any other action.

In Pokémon XD: Gale of Darkness, Shadow Lugia's Heart Gauge is fully closed until it is placed in the Purification Chamber with all nine chambers at max tempo. When this is done, the Heart Gauge will automatically open and Lugia can be purified. However, if Lugia is taken out before being purified, its Heart Gauge will again close completely.