Water Seer class series

The Water Seer class series is the default class series available to the Mercury Adept Mia and her son, Rief. They are considered members of this class series if they either have only Mercury Djinn set to themselves or no Djinn whatsoever. In the GBA games, this class series is unique to Mia; the other Mercury Adept, Piers, is a warrior rather than a healer and receives his own default class, the Mariner class series. The second Mercury Adept in Golden Sun: Dark Dawn, Amiti, also has his own class series, the Aqua Squire class series.

The class series, when at its highest stage of Angel and compared to the other series at their highest respective stages, features good PP, but lower Defense, below average HP (tied with both Oracle variations and the Necromage and higher only than the Sorcerer), low Attack (tied with the Wizard and Necromage and higher only than the Sorcerer), and very low Agility (tied with the Beast Lord and higher only than the Hero). This series' Luck rating is tied with the Dragoon class series and the White Mage class series as having the highest luck rating a class series can give. Noteworthy Psynergy from an endgame perspective includes Pure Wish, Pure Ply, Cure Poison, and Break, and the heaviest area-of-effect Psynergy it has is Ice Missile. Average stats from an endgame perspective are 142% (144% if you don't factor in Luck).

Psynergy Setup
Golden Sun & Golden Sun: The Lost Age

Golden Sun: Dark Dawn

Analysis
The Water Seer class series, Mia and Rief's mono-elemental class, is the resident healer of the monoelemental classes. With access to both the Wish series and the Ply series, the Water Seer class can do both single-target and mass-healing, making them one of the more versatile healers. Angel class has the strongest group healing in the game before equipment and summons due to having at least 30 points of Mercury Power more than any other highest stage class with the Wish Series. This boost by itself contributes 120 HP to every Pure Wish (or 140 HP if Mia or Rief has 9 Mercury Djinn set). Meanwhile, the Ice series is a fairly powerful offensive series. The Water Seer's high PP pool means they can make full use of all their Psynergy.

However, the Water Seer has several drawbacks. Their low Agility stat means the Water Seer will either be last or next to last to move. In addition, their offensive options are fairly weak in the later parts of the game, until Ice Missile is learned. The Water Seer's only has two offensive Psynergy lines, while the sub-par attack means physical attacks will do little damage. In Dark Dawn, the lower powered Frost series is switched out for the higher power Douse series. While the lineup is still limited, the Ice Queen Stone can help remedy this, by granting the Cold Snap Psynergy series.

Keeping Mia and Rief (and everyone else for that matter) in their Mono-elemental class will get the job done without much fanfare. However, switching them to either a White Mage or Hermit will sacrifice their Ply series at the gain of some Agility, more PP, and a much stronger offensive Psynergy lineup. Alternatively, switching them to a Guardian will grant them the Revive Psynergy while retaining all their old healing Psynergies, as well as increase their HP, Attack, and Defense slightly and give them the Cutting Edge/Plume Edge Psynergies, but all at the cost of a drastically lowered PP rating.