Elsirelle is a Light, and primarily slash type, damage dealer that can be a good addition to mono Light teams, partly due to her master ability, and the other being a potential chainer.
With Nightblade as her main job, she's able to provide a small area team buff that increases pierce and missile resistance (Double Resist), and another for slash attack and defense penetration (Concentration). Her damage skills aren't high compared to other physical damage dealers, but what makes her shine is her Limit Burst. It deals damage and reduces AP consumption by a moderate amount, letting her use skills more often like Drain Rush for chaining/healing and AP Drain for some self sustain.
Her Spellblade subjob comes with some essential skills that can make her into a viable magic tank (Resist Magic + Magic Barrier + Taunting Spell), but there aren't many benefits for making her into one, especially since she can't equip cloths for Smart Coat, but maybe Drain Rush for some self sustain. Her Dual Gunner subjob, on the other hand, gives her some needed ranged skills, but lacks decent buffs towards missile attack.