Coal

Coal is a Mars Djinni found in Golden Sun:  The Lost Age.

Coal is the fifth Mars Djinni in Golden Sun: The Lost Age. The separate Djinn list viewable with the Select button from the status screen lists Coal and all other Mars Djinn introduced in The Lost Age before the seven Mars Djinn from the original Golden Sun, most likely because all the Djinn from the original game can be gotten all at once late in The Lost Age. However, the original Djinn are  ordered before all of the Djinn introduced in The Lost Age in the Djinn inventory screen while they are allocated to characters. By this order, Coal is the twelfth Mars Djinni in the GBA series, rather than the fifth.

Basic description

 * "Rally your allies to boost agility."

When Set, Coal increases its Adept's base HP by 11, base PP by 3, and base Agility by 3.

When Coal is unleashed in battle, all currently active and alive Adepts get their Agility ratings temporarily buffed to +100%, the maximum agility bonus possible. It doubles everyone's speed, in other  words.

In The Lost  Age, Coal's unleash animation visually resembles a glowing red image of a Mars Djinni floating above the party while collections of small, glowing red energy orbs gather into each Adept in order, and then each Adept is surrounded with a brief red windy effect to indicate a speed boost.

Location
Golden Sun: The Lost Age: Coal is acquired in a cutscene in Izumo. You first have to have completed the Gaia Rock dungeon, including defeating the Serpent, then triggered a lengthy story cutscene in Izumo with Lady Uzume, Susa, and Kushinada. Following that, you may present the Dancing Idol key item that you had found and made use of in Gaia Rock to Uzume, and through another cutscene she exchanges Coal - a pet that has apparently become very dear to her - for the dancing doll of Mt. Mikage.

Analysis
General: Using a turn to double all Adepts' agility ratings basically solves all possible issues of each one of your Adepts not moving before the enemy in a given turn, for several turns. Depending on the boss, this can be highly useful, though it can be argued that a Djinni that halves all enemy Agility ratings is the same, such as the Venus Djinni Mud. There is one advantage the latter has over doubling party agility, though; since the agility boost counts as a buff, it can be removed by the Break Psynergy if the boss has it (such as Sentinel). But a Djinni that halves the enemy's Agility is something that no enemy in any game can dispel from themselves. In Dark Dawn, identical equivalents to Coal are the Jupiter Djinni Fleet and the Mercury Djinni Torrent.

By game
Golden Sun: The Lost Age: Coal is the only effect that increases agility in any way, and is the one Djinni that doubles everyone's speed, for much of the game's middle segment. This makes it very useful, perhaps life-saving, to use against bosses from Avimander up until Karst and Agatio, allowing you several turns where your party can safely attack and heal themselves before the enemy gets to take its action(s) those turns. When Isaac's party joins Felix's party along with their returning Djinn, the Jupiter Djinni Zephyr becomes a second Coal, and these two together remain the only form of Agility enhancement in the series. They continue to be extremely practical if the player decides to use them specifically during boss fights where the boss happens to be faster than several Adepts and that proves to be a problem.

Name Origin
Coal is a readily combustible black rock used to start small fires.

The manual to the North American version of The Lost Age incorrectly names this Djinni "Turbo".