How about a "sadistic/psychopath loli"? Someone who is loyal and only listen to the MC. Have her act cute around us but merciless in the battlefield. Someone who will say stuff like "will you please kill yourself for me, onii-chan?" to the enemy. With a scene of her singing a children's song while slowly torturing someone to death. Show a CG of her covered in blood while smiling cutely as she asks us to compliment her and pat her in the head...
Okay that turned dark really fast....
I don't know if a character like that will fit or anything, especially as a squad leader.
Maybe as a new party member or something? Heck you can even make her one of the children after the time skip.
It's up to you whether you want her as a "little sister type" or a "daughter type". But I think a daughter type will clash with Kayelinth, especially if you put her in the "present time" before the time skip.
So what do you think?