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
Mia and Rief's Mono-elemental class is the resident healer of any group. The Wish series will get the job done and is invaluable for boss fights, while the Ply series is useful to patch up one Adept who desperately needs it. The biggest issues, however, are a debilitating agility stat, meaning Mia and Rief will quite often be taking their turns last (or close to last), which may or may not be beneficial depending on how you look it; the lack of a revive skill and a bare bones offensive Psynergy lineup, which when coupled with the sub-par attack rating, makes using them in regular fights (first game aside, when Mia is the only Mercury Adept) a rather unappealing prospect.

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.

It is worth noting that, for all its shortcomings, the 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).