Fever

Location
Fever reposes in Imil Falls Cave, the tiny cavern at the town of Imil, reachable by sliding around through the frozen river and into the hole of the frozen waterfall. To reach there, first you have to stand in front of the snowman at the north end of the town and cast Move to shift it left and drop it onto the frozen ice river below. Then entering the river via the opening just to the right of the sanctum in the southwest part of town, slide right, up, left, up, left, up, right, up, left, up, right, and up to enter the opening. You automatically slide into Fever, and press A to immediately add it to your collection.

Description
When Set, it increases base HP by 8, base Attack by 3, and base Agility by 2.

When Fever's battle effect is used, the user attacks a target and an orange cloud is let loose upon impact, dealing a Mars-based attack equal in power to the user's normal physical attack with an additional 30 damage points added to the result. There is then a chance that the target will be afflicted by Delusion. The Mercury Djinni Fog from The Lost Age is identical to this other than in terms of element.

Ability analysis
In Golden Sun, Fever is the weakest of the three available offensive Mars Djinn from an endgame perspective; like Torch, it only adds 30 damage points, but unlike Torch, the added status effect of Fever, the Delusion status condition, is considered to be rarely helpful to have inflicted on enemies because Delusion's associated accuracy drop does not apply to anything that is not a normal physical attack. Fever is quite useful to make use of early on in the game when it is first acquired, though, for at that point 30 additional damage is quite substantial. Fever is a Djinni whose effect becomes less valuable over time when your focus is using only stronger Djinn to conduct and win battles with, such as the stronger Scorch.

In The Lost Age, when Isaac's party joins Felix's party along with their returning Djinn, Fever becomes all the more obsolete as a method of attack you would use if you want to make battles won easily, particularly because another Mars Djinni exists whose battle effect is an interesting upgrade on Fever; Shine, found in Contigo as soon as the party joins, which adds 60 damage and has a chance to inflict Delusion on all enemies, including the ones not specifically targeted by the user of Shine. Whether even that Djinni is used in battle is another story, but it certainly obsoletes Fever on its own.

In Golden Sun: Dark Dawn
Fever is confirmed to return in the game from the E3 2010 floor demo.

Name Origin
A fever is a symptom where the body's internal temperature rises above normal.