Foam is a Mercury Djinni found in Golden Sun: Dark Dawn. It is southwest of the city of Tonfon, at a small area of forest at the base of mountains, just inside the perimeter of the darkness of the Grave Eclipse.
When Foam is unleashed in battle, all enemies' Agility ratings are lowered down to 50% of their normal values, the lowest value the Agility rating can be debuffed down to.
In Dark Dawn, Foam's unleash animation visually resembles the user summoning the 3D model of Foam above the enemy group's heads in a large blue flash. Foam then flutters above the enemy group as a series of large white snowball-like mounds of foam appear around the enemy and falls to the ground and disappear.
Golden Sun: Dark Dawn: Foam is one of the Djinn that is only found by being randomly encountered in a specific area of the overworld map that otherwise appears to be featureless scenery. It is at a small area of forest at the base of mountains southwest of the city of Tonfon, just inside the perimeter of the darkness of the Grave Eclipse. Foam must then must be battled and defeated to be acquired.
General: Foam being an effect that halves all enemies' agility ratings can be thought of as equivalent to an effect that doubles the whole party's Agility ratings, which other Djinn can do. However, there is one additional benefit that halving enemy agility has: if an enemy has the Break Psynergy, they can't remove their own statistical drops the way they can remove a party's self-inflicted statistical buffs. This essentially lets the entire party move before a particular boss enemy, rather than only part of the party, while the other part of the party moves after the boss' action(s) the current turn. This can be situationally useful in letting an Adept with a group-healing effect use their healing before a boss can fell an Adept low on HP.
Golden Sun: Dark Dawn: In general, Agility-modifying effects may not be as useful in Dark Dawn as in previous games because with equipment like the Zol Ring, your party is more likely to move before your opponents anyway. This makes Foam one of the less useful Djinn in the game. However, it may be useful for lowering the agility of a Guardian Ball in the Star Magician fight, so that in the following turn, it can successfully have Ivy or Doldrum used against it to prevent it from using its Guard Aura and allow the party to safely unleash a salvo of summons on Star Magician's full group. Foam's battle effect is identical to that of the Venus Djinni Vine, and the two can be used interchangeably.