The Miko class series is the default class series available to the Venus Adept Himi. Himi becomes this class series when she either has only Venus Djinn set or no Djinn at all. This class series is unique to Himi; Matthew, the other Venus Adept, is a warrior rather than a mage and has his own default class series, the Squire class series (shared with his father and uncle).
The class series, when at its highest stage of Radiant Miko and compared to the other series at their highest respective stages, ties with the Sorcerer for having the lowest HP, Attack, and Defense ratings, but ties with the Wizard, Dark Mage (Medium class series), and Necromage for the highest PP rating, and has high Agility. This series' luck rating is high. Noteworthy Psynergy from an endgame perspective includes Potent Cure, Nettle, Weapon Grace, and Revive, and the heaviest area-of-effect Psynergy it has is Reigning Dragon.
The Miko class series is very similar to the Wind Seer class series. Both have very low HP and Defense, making them soft targets, and both have low Attack, making them poor attackers. The Miko must rely on Psynergy for offense. The very high PP pool ensures that this is not a problem, despite the fact that Reigning Dragon ties with Thunderhead for being the most expensive Psynergy in the game. The high Agility stat allows the Miko to go before most enemies. Potenet Cure and Revive make the Miko a good healer, a bonus that is compounded by the high Agility.
However, despite having a powerful psynergy pool, the Dragoon class series (assuming you give Himi a Zol Ring) and the Ninja class series, although they have less powerful psynergy pools, possess more well-rounded stats, which increase her durability and attack, giving her the combat skills she would lack as a Miko. So while the Miko possesses a powerful psynergy pool, there are different class choices with less psynergy-power but more offensive skills.