I'm confused why we're still pretending to honor spoilers in this thread, seeing as how Cone has pretty much blabbed about everything at one point or another, but...
The last character you get is the opposite of Melia in many ways. Most of her damage comes from auto-attacks, which (when properly used) she can dish out at an unfair rate. Prioritize her Speed Shift art, activate it, and learn to enjoy Chain Attacks to their maximum potential. Equip her with Haste and Double-Attack gems, concentrate on her Skill Tree that maximizes her critical attacks (and then link other characters to those skills too!), skill link her to maximize her agility/Shulk's Chain Attack skills, then sit back and enjoy.







