Alice the 15th is a returning character from the original trilogy. She was reported dead when she left to explore the Remina Tartarus and didn't return even after almost twenty years. However, after the precipitated departure of Alice the 16th, she has seemingly returned from the grave.
Before the beginning of the game, a few years after the appearance of the Tartarus and the birth of her daughter, she snuck out of the Monster Lord's Castle to explore the one located where Remina once was. After a few years, Tamamo took upon herself to raise Alice the 16th as her own daughter to have her claim the throne instead.
She reappears after the departure of the 16th, as one of the three new factions that arose in its wake, who all have the intention of claiming the throne for themselves. Alice the 8th, Alice the 15th, and Alice the 17th.
The 15th took control of the 16th's army after her disappearance, including her previous supporters and the Four Heavenly Knights; thus she has the largest army, but she's not the most powerful individually. However, she is said to be leading the Lilith Sisters, a trio of succubi who are extraordinarily powerful.
She's brought up again at the second camp scene on Alice's route. Alice is upset that her mother would have openly abandoned her and claimed her army. Of course, she has every right to do so, but the small sealed Monster Lord is unsettled that her mother hasn't contacted her since or even tried to.
Luka asks Alice how exactly her mother went missing if she is comfortable with answering that. She does, and she reveals that her mother was not slain by the group of heroes like in the original trilogy, but instead disappeared and was last seen heading in the direction of the Remina Tartarus. She never returned. The 15th was presumed dead, and after a year, the 16th was crowned Monster Lord. Alice was still very young at the time as it happened more than ten years ago. However, she has seemingly come back from the dead, suspiciously just after White Rabbit sealed Alice. The duo realizes that since the XVth has every one of the XVIth subordinates under her, so it's likely that she's the one who sent Granberia after Neris.
She is mentioned for the final time in part 1 after Luka defeats Astaroth in Luddite Village. The red-haired succubus reveals that Alice the XVth is aware of her daughter's plight and want her out of the way of her plan. The succubus also incidentally inform everyone that while they're allied with the XVth, they're actually serving the Dark Goddess herself in another world.
Later, when the party has solved the mystery behind Grangold's aggression by removing Lilith and Tamamo No Mae, she finally appears directly when the party is enjoying a well-deserved feast. A projection of her declares war to the four human kingdoms with the intent to annihilate them and their monster allies. The feast is interrupted by the shocked party as people ready for war. The leaders are even more dumbfounded when no army show up the very next day, contradicting some of the most basic tenants of war strategy, something that has certainly been taught to her by Tamamo.
Her final appearance on Alice's route was at the end of Part 2, when Morrigan brings the news that Marcellus has killed Alice the 15th in battle, showing him as he walks away. On Ilias' route, her situation is unknown as Morrigan isn't with the party.
- Her relation with the White Rabbit is unclear. The latter acts on her own, but her task of sealing Alice fits too well in the XVth plan to be a mere coincidence. Furthermore, the whimsical concept made flesh never steps in when the Lilith Sisters or the seraph are present, even as she tries to eliminate Neris for not being part of the true history.
- Her relationship with the Lilith Sisters is just as foggy. The latter group regularly present it as an alliance of interests with their goals aligning. However, her behavior and goals are so different from the original trilogy that even this alliance is questionable. Since the Lilith Sisters are backed by the Dark Goddess from their world, it is likely that the latter is the one who convinced the XVth. By the end of part 2, the one thing that is clear is that said goal is the destruction of the world in a way that is as close to the potential true history as possible to transfer the souls of the deceased to the Monster World and save them.
- Again, the reason she took so much time to return, which led to her daughter being left alone in Tamamo's care, is never explained and unclear. The only thing obvious is that, just like Marcellus abandonning Luka, this fact aligns on true history.