Jill
Jill plays a vital role in the story from the start of the game through to the end. While many mistake her for being Clive’s sister, she’s not related to either Joshua or Clive by blood and instead is a ward of the Northern Territories. In fact, it’s strongly implied that Jill is from Mysidia or the neighbouring areas, which, given her light grey colored hair, makes her appear to be from that same area.
You can’t control Jill as a character, but she does join your party at several points in the game. You also battle against her (or, as she’s referred to in that chapter, Shiva’s Dominant) early on in the game. While she’s in your party, she will attack with her sword and launch ice-based magical attacks from a distance. She’s very helpful for tying up one or two enemies so that Clive doesn’t get overwhelmed by attacks.
Just like Joshua, she will permanently join Clive for the remainder of his adventures in the final chapter of the game when you complete the 
Priceless Optional Quest. Use the link below to find out more:
Priceless
Optional Quest
This quest begins from a missive called 
Concern for Jill.
“Joshua is worried that recent events have begun to weigh heavily on Jill and wonders if there is not something he and Clive might do to lift her spirits.”
In-Game Description
*Spoiler Alert*
“Dominant of Shiva and trusted confidant of Clive, leader of the new hideaway. Born a princess of the Northern Territories, Jill was made a ward of Rosaria after her father the Silvermane’s incessant raids were quelled by the duchy. She entrusted her Eikon to Clive after he was bested by King Barnabas, that he might have the strength to fight their fate to the last.”