Kilphe (Summer) is a Light bruiser that's equipped to handle mages. Base resistances, master ability, and active buff (Spirit Ward/Spirit Ward Mastery) gives her some tankyness towards magic damage, and being an Axe wielder + the jobs she has available gives her high damage with some penetration towards DEF and Slash. What makes her truly unique is that she's one of the few units that can inflict Freeze/Frostbite debuff, which can decrease the enemy's AP gain.
Having access to thief job is a pretty nice bonus since it comes with a +1 move passive option, and there's also Steal Time that's still useful against most bosses. Her EX abilities might not be crucial, but the additional effects are quite notable and worth considering since it gives her some self sustain.