If Flaming Fist Marcus has been prevented from capturing Isobel and destroying Last Light Inn, and if the Nightsong has been freed from her cage to wage war against Ketheric Thorm at Moonrise, Lilineth will take the place of Harper Lymn as a failsafe in the event that the latter has been killed before the Assault on Moonrise Towers. Speaking to Lilineth will have her mention that a lot of Harpers had lost their lives in the first war against Ketheric, and that this time she intends on putting an end to him for good[1].
After the main battle, if she manages to survive the fight, she will guard the first floor of Moonrise Towers with the remaining Harpers.
If killed during the Assault on Moonrise Towers, a replacement Harper failsafe (either Grenywald, Lanni, or Olys), or a generic Harper Archer will take her place as a Silver Harp Squadron summon.
Lilineth has four level 1 spell slots, and two level 2 spell slots, despite having no spell to cast.
Unlike Lymn, who is unable to attack twice during a turn despite having the Extra Attack feature, Lilineth is not affected by this bug.
References
↑"A lot of Harpers died stopping Ketheric the first time around. We'll make it stick this time." - Lilineth