She is a fortune teller that trains to be as good as her sister, Sapphire.
She is recruitable at the southeastern house at Loanne, neglect her proposal to a fortunetell, she will join you.
Ruby is a decent character, and a competent mage(23 Magic), but her physyical statuses aren't bad(17 in both Strength and Dexterity), meaning that she easily follows a battlemage route.
Clubs, Staves and Axes are most reccomended to her, as all types of sword, guns, bows and bare fighting uses Speed for the best arts, and you probably have a decent spear user like Primiera, Harid, etc.
Ruby fills a good role as a mage, but can also use Shell Destroyer decently.