She is recruitable at Wilminton Weapon Shop with Gustave XIII in party.
Leslie is quite disappointing, as she is very slow(13 Speed), but has high Dexterity to make up(22). Her magic score is good(21), fitting her the role of a support character.
Now to weapon choice, I would suggest guns, as none of their arts don't require high speed to do decent damage.
Leslie is a good starter mage, even though she hasn't a cosplay item...