Valkyries have High ATK and HIT, moderate DEF, and poor RES and SPD. The Valkyrie sports slightly higher HIT and SPD, as well as lower DEF, than the Warrior; but otherwise there is little difference between the two classes statiscally.
The Valkyrie makes use of the Sword, Spear, and Bow as her weapons of choice, the best being the Spear. (As she learns all of its skills, the other weapons she learns up to Rank 5) As such, she can operate at any range and be useful. Of course, like most physical classes, her low RES and elemental Resistances make her easy prey for Magic-casters.
What seperates the Valkyrie from the Warrior are her Evilities. While the Warrior encourages direct offense, the Valkyrie's Evilities are more defensive.
Her native Evility, Assault Strike, increases the damage she deals by the amount of Panels she has moved in a turn (making Shoes viable Accessories!). Armor Mastery meanwhile cuts the damage taken from normal attacks by half.
(Note: in A Promise Revisited, her native Evility is mistranslated to "Number of enemies defeated" instead of panels moved. However, Valkyries still get damage boost based on the Panels moved.)
The Valkyrie is an open-ended attacker that has many uses in many areas.