Exspheres are very important in the plot of Tales of Symphonia. Exspheres serve as power-enhancers for the party and antagonists, the Desians, and ultimately Cruxis. It is a lower level version of the Cruxis Crystals, which are used by members of Cruxis.
A Key Crest is a work of dwarven technology designed for use with an Exsphere. When used correctly, the Key Crest inhibits the negative effects of the Exsphere. It is made of a special Inhibitor Ore which can only be processed by a dwarf. Usually, the Key Crest acts as a mount for the Exsphere, but an accessory made of Inhibitor Ore with the correct charm carved into it will suffice. An Exsphere acts as a parasite in its host, and can be life-threatening without a Key Crest. If an Exsphere without a Key Crest is removed, the host's mana deteriorates. Victims of this incident include Marble and Alicia Combatir.
Exspheres give their users special equitable skills known as Ex Skills, which give many additional status effects. Ex Skills can only be equipped on the current Ex Gem the player has equipped on the character at the time. Ex Skills can be used to boost a character's attributes such as speed, strength, and hit points. In battle, Ex Skills can be used to add attacks in combos and decrease spell casting time. In addition to the normal Ex Skills, there are also skills called Compound EX-Skills, which are a result of combining normal Ex Skills.
Tales of Symphonia
Originally, Exspheres were thought by the main characters to be simply objects that increased the user's fighting and survival potential far beyond his or her maximum limits. However, it is learned in Kvar's Human Ranch that Exspheres are made by Desians by cultivating humans and turning them into Exspheres. This revelation nearly results in Lloyd Irving discarding his Exsphere, but Kratos Aurion convinces him otherwise. The party later discovers that Exspheres are not just in Sylvarant, but in Tethe'alla as well. Most Exspheres in that world are taken from the Toize Valley Mine, though their origins were the same. Most people with Exspheres in Tethe'alla are given to them by the Renegades or Vharley, an Exsphere broker. Expheres are often used as an energy source for the machines in Tethe'alla.
Tales of Symphonia: Dawn of the New World
Sheena Fujibayashi and Regal Bryant explain the King of Tethe'alla collected all the Exspheres in Meltokio after the events of Tales of Symphonia. Several of the original characters still be wear their Exspheres during the story, but Sheena explains eventually after everyone finishes their assigned missions, they will have to return their Exspheres.