Alice the 15th is a returning character from the original trilogy. She should be dead, but after the disappearance of Alice the 16th, she has seemingly returned from the grave.
She is brought up for the first time by the chief elf in Enrika.
The 15th took control of the 16ths army after her disappearance including her previous supporters and the Four Heavenly Knights; thus she has the largest army, but perhaps not the most powerful individually. However, she is said to be leading the Lilith Sisters, a trio of succubi who are extraordinarily powerful.
She was brought up once again in the second camp scene, assuming you brought Alice.
In the camp scene, 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 Alice 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, meaning these events would have occurred at least fifteen years ago. But she has seemingly come back from the dead. Suspiciously, just after White Rabbit sealed Alice.
After the 17th had started causing all of the commotions, it was likely that the 15th was the one who sent Granberia to deal with her. Since the Heavenly Knights are under the 15ths control, only she would have the authority to order them to engage Neris.
Two things are revealed in this scene. For one, it is indeed true that the 15th is ignoring her daughter for a reason, and it isn't simply because she is unaware of her. Astaroth:"Alipheese the 16th... you wouldn't understand. That is why your mother removed you from the plan." In the same scene, it is confirmed the Sisters are Alice the 1st's subordinates. Astaroth: "Well then, let's meet again. I hope that you, too, work your best for our Mistress, Alice's sake..." Mistress Alice is confirmed to be the first by Ilias, "She said "Mistress Alice"... that's Dark Goddess Alice's..."
Alice's mother later declared war and sending a message by some holographic projection of her that she will lay siege on Grangold, Sabasa, Grand Noah and San Ilia shocking the party.
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 as he walks away ages after Marcellus helps his son Luka and dragged Adramelech to another dimension so that Luka and his friends can get away. On Ilias' route, there wasn't such news since Morrigan is not the party. If she was killed or not on this route too was left unanswered.
- There is a lot to be speculated here. Astaroth states that the 15th "removed her from the plan". Of course, this was done by the White Rabbit, who sealed her. 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. I wouldn't be surprised 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 they're 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?