I know this is supposed to be the outers with Princess Serenity, but…
Look at her moon crescent symbol, and her dress with the bow at the very top. Could this, perhaps, be a young Queen Serenity?!
What if the outers had been her personal guard when she was younger? The Moon being taken care of by the big three: Sky, Oceans, and Afterlife? It was the four of them on the moon, living and growing together, learning together, the three protecting their beloved little princess as they developed as senshi.
And when they were all fully grown, they were there for her as she ascended the throne as Queen Serenity and were so proud and supportive and so genuinely excited for the future of the Solar System with their beloved Serenity leading the way. It was perfect.
As time went on, Queen Serenity became pregnant, and her three guardians were ecstatic, already planning activities for the new princess, dreaming up tiny little gowns and party ideas, and even deciding to awaken the other planets to serve as the Princess’s guardians. One by one the other planetary guardians awoke, all waiting for the Queen’s daughter to be born; Uranus, Neptune, and Pluto taught them even from their youngest age, about the joy of protecting someone you love and cherish.
And then something terrible happened.
One of the newly awakened senshi had a dark and terrible power, a power that could destroy their world and all of its people in one fell swoop: Sailor Saturn. Though she was young, like the rest of the Princess’s future guardians, already the three saw the potential within her to cause a great and horrible danger to their beloved Serenity and Serenity’s future child. One misstep on the girl’s part, one wrong move, and everyone’s life would be forfeit.
They convinced their Queen to seal Saturn away, for the good of the people and the safety of the new Princess… and they decided to go with her. Condemning a senshi to such a fate weighed heavily on their consciences, and they knew that sealing someone away wasn’t enough - they needed to provide some kind of lock, to have her awakening controlled by specific circumstances that they could control, by keys: their talismans. When their talismans came together again, Sailor Saturn would awaken, and the Silver Millennium would fall.
The three waited until after the Princess’s birth to leave, so that their Serenity might not miss them as much; and to hold a picture of the new Princess in their hearts to remind them in their future times of loneliness just why they had made the decision, why they had sacrificed their future for fear of awakening the destruction.
They retreated to their castles quietly, their final farewells naught but nods of respect, love, and an understanding of their new purpose. While the other planets would protect their little Princess and their beloved Serenity, they would watch the kingdom from afar, protecting it from outside threats as they guarded against the awakening of the messiah of silence. And while they would never be able to leave their castles, they all knew the vision of their Queen and Princess would keep them company for the rest of eternity.