The Aqua Squire class series is the default class series available to the Mercury Adept Amiti. Amiti becomes this class series when he either has only Mercury Djinn set or no Djinn at all. This class series is unique to Amiti; Rief, the other Mercury Adept, has his own default class series, the Water Seer class series.
The class series, when at its highest stage of Aqua Master and compared to the other series at their highest respective stages, has high HP, good Attack, but below average Defense, PP, and Agility. This series' luck rating is high. Noteworthy Psynergy from an endgame perspective includes Diamond Berg, Pure Ply, Cure Poison, Break, and Avoid, and the heaviest area-of-effect Psynergy it has is Megacool. Average stats from an endgame perspective are 143% (148% if you don't factor in Luck). Amiti will only learn Frost (Cold Snap) if equipped with the Ice Queen. He will learn the Frost Psynergy series without levelling up. He will learn the Douse Psynergy series by levelling up.
Keeping Amiti in his mono-elemental class is nearly identical to Piers, with Jenna's equipment setup. He has one of the highest HP ratings in the game, which partially compensates for his low defense. His PP rating could be considered poor at best, and will be quickly strained by Diamond Berg and Megacool. The high attack rating compensates for not being able to wield warrior-type weapons. The below average agility rating ensures that Amiti will take his turn after most characters.