A Mars Djinn
Mars is also known as, 'The Goddess of the Fire'. In Golden Sun, she is the embodiment of fire. Garet, Jenna, Saturos, Menardi, Karst, and Agatio are all of this element. The Mars element is connected to the Venus element, which Saturos and Menardi explain after lighting Venus Lighthouse to regain their energy. It is also why Psynergy flowed into the parents of Isaac, Felix, and Jenna during the ending of Golden Sun: The Lost Age despite the fact that they might not have been Mars Adepts, but Venus Adepts instead.
The Mars Summon is gained automatically upon obtaining one Mars Djinn. It's appearance is a Mars Djinni showing up and ramming itself into the enemy, creating sparks. If this is the first summon used by a character in a battle, then that character's Fire Power will rise by 10 for the battle.