As a summoner, Sheena can make pacts with various Summon Spirits that live in Sylvarant and Tethe'alla. Each has an elemental affinity. Though they appear to have genders, they are actually genderless. Sheena can only summon Summon Spirits while in Overlimit Mode.
Aska is one of the two summons associated with light. The name "Aska" most likely comes from the Japanese word "Asuka," which means "bird" and "to fly." Aska can be found at the Tower of Mana in Sylvarant (after a long and annoying fetch quest with trees and flutes or some crap).
Celsius is the summon of ice, whose name comes from the word "celsius," which is a unit of measuring temperature named after Swedish astronomer Anders Celsius. Celsius can be found at the Temple of Ice, near Flanoir in Tethe'alla.
Gnome is the summon associated with Earth. Gnomes are small, mythical creatures that are said to dwell underground, thus associating them with the element of earth. He is found in the Temple of Earth in Tethe'alla, which is near Meltokio.
Efreet is the Summon Spirit of fire. "Efreet" comes from Arabic mythology, where Efreet (also spelled "Ifrit") is a Jinn (a superhuman mythical creature) that embodies fire. He is found in the Temple of Fire in the Triet desert, in Tethe'alla.
Luna is one of the Summon Spirit of light, along with Aska. Luna is the Roman goddess of the moon, and her Greek equivalent is Selene (which was originally the name of Sylvarant's moon, according to a book in the Tower of Mana), whose name comes from the word "selas," which means "light." "Luna" in Latin means "moon." She is found at the Tower of Mana in Sylvarant.
Maxwell is the summon associated with atoms or space/time... or something. He is the "ruler of the elements." His name probably comes from the physicist James Clerk Maxwell, who studied electomagnetism. Also, the prefix "max-" can mean "great." Maxwell is found in Exire, which floats above Tethe'alla. He can only be accquired if Sheena has made pacts with all the other Summon Spirits.
Origin is sort of like the leader of the Summon Spirits. He embodies "heaven, earth, and everything in between," according Sheena's incantation. The name "Origin" is obvious, meaning "the point at which something comes into existence." He is found in Torrent forest, following Lloyd's battle with Kratos.
Shadow is the Summon Spirit of Darkness. He scares me ;_;. But that's not important. Ahem. His name is rather obvious, "shadow" means an area that is not illuminated. Shadow is found in the Temple of Darkness, which is located near the Fooji Mountains in Tethe'alla.
Sylph is the collective Summon Spirit of wind, made up of three fairy-like critters: Sephie, Yutis and Fairess. "Sylph" is a mythological creature, similiar to fairies or nymphs, associated with air. They are found at Balacruf Mausoleum in Sylvarant.
Undine is the Summon Spirit of water. "Ondine" is a water nymph in German mythology, also spelled "Undine." "Undine" comes from the Latin "unda" which means "wave." She can be found at the Water Temple, at Thoda geyser in Sylvarant.
Volt is the Summon Spirit of lightning. He speaks in a different language, which is why Sheena intially failed to make a pact with him. "Volt" is a unit of measuring electricity. Volt can be found in the Temple of Lightning, which is off the coast of Mizuho in Tethe'alla.
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