Harley is seen as the most useless character at the message board for the game at GameFAQs because she has a poor equipment selection and useless abilities.

Here are her abilities:

Fight
Piercing Sight
Gil Toss
Item

Harley only has access to whips, rods, and mystic armor, much like Rydia. Her Strength, Vitality, Intelligence, and other statistics more closely resemble that of a Magician, so she will never be a strong attacker because of the low attack power of whips.

Piercing Sight is designed to cast Dispel on a single enemy and assign them a weakness like the Debilitator in Final Fantasy VI. Its success rate leaves something to be desired, and it will not work at all on bosses or special enemies.

Gil Toss is much less powerful than its counterparts in other games. It does have defense-ignoring properties, at least.

Calca and Brina, at the very least, have more useful abilities even though they are more random.