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 after a year, but after the disappearance of Alice the 16th, she has seemingly returned from the grave.
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, 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 reappears when the party enjoys 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, contradicting some of the most basic tenants of war strategy, something that has certainly been taught 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.
- There is a lot to be speculated here. Astaroth states that the 15th "removed her (Alice the 16th) from the plan". Of course, this was done by the White Rabbit, who sealed Alice. Wouldn't that mean effectively that White Rabbit is at the least working together with the 15th? They do possess very similar goals, keep this parallel world as close to the fundamental reality as possible. Now the White Rabbit is guiding the 16th down a path. Perhaps this was also a request of the 15th?
- If the 15th sent Granberia to deal with the 17th, that would mean they are not allies, and the 15th has no idea of 17th's power if she believed Granberia would beat her. Instead, they both fought to a stalemate, not to mention that Neris was already exhausted, and she wasn't permitted to reveal her true power. Also, it is likely 17th wasn't trying to beat Granberia. After all, Granberia was a good friend of the 17th's father.
- Why would the 15th suddenly abandon her daughter? She went off to the Remina Tartarus and never returned. It is a similar story to Marcellus; he left Luka while he was at a very young age to stop the chaos spreading. A similar time to when the 15th did. And they are both parents of Alice and Luka. Perhaps, an event occurred which made them aware of what was going on outside of their world, or someone like White Rabbit visited them and explained the current situation. It wouldn't be surprising if Marcellus and the 15th weren't somehow connected. The White Rabbit is guiding both Alice and Luka down the same path. And every step of the way, they learn more about what their parents are up to.
- As for why she went to the Tartarus, it was likely for the same reasons Marcellus did. To save the world.
- Finally, it is implied by Astaroth that they are working with the first. If the sisters are allied with the 15th, that would mean that the 15th is also serving the first. After all, why wouldn't she be?