Okay so something I think the Percy Jackson fandom often overlooks is the fact that after the war with Kronos, part of Percy’s promise with the gods was for the Big Three/no kids pact to be broken.
So what if Poseidon had more kids, but was hiding them in the deepest depths of the sea, kind of like how Hades hid the Di Angelo siblings in the Lotus Casino? What if Percy was chosen by Poseidon to be sent to Camp Half Blood, but only because he was the oldest out of the kids and destined to be the most powerful?
It would be very possible for Percy to come home from the war with Gaea to a bunch of brothers and sisters, thus these headcanons were born
•So after a terrible, long day of fighting Gaea’s forces relentlessly, Percy trudges dejectedly into his cabin really late that night ready to collapse on his bed but is greeted by a bunch of black-haired green-eyed kids chilling on his bunk like “hey”
•He drops his sword and doesn’t know whether to be shocked, overjoyed, or horrified
•The pieces click together in his mind and he believes that after all his fighting and prophecies Poseidon has blessed him with a group of wonderful children to call his siblings like he always wanted and tears stream down Percy’s face as he stares at them
•There are five in all, two girls, three boys
•He finally shouts with happiness, runs over, and hugs them all in a giant group hug wailing joyously “I have brothers and sisters! I have brothers and sisters!”
•The kids are confused and a little freaked out at first but then they realize this is Percy Jackson, their brother, the brave, wonderful hero that everyone has been talking about ever since they arrived, and they smile and laugh and cry and hug their brother
•They all befriend Percy instantly, and are earnest to learn everything about him
•So Percy tells them EVERYTHING about himself, starting from his time at Yancy Academy and Smelly Gabe when he was 12 to his battle with Kronos when he was 16 to his 8-month slumber from Hera and the new prophecy with Gaea and how he played a part in it
•They are awed with their brother but then Percy demands to know their stories as well. They explained how they were all hidden in the ocean up to a few months ago when they arrived at camp, where they all met one another and decided to swallow this whole being-a-half-blood-thing together
•The first girl is only four years old. Barely a week after Poseidon sired her, he took her to a beautiful mermaid’s palace where she grew up and waited out the war (Percy thought this story was a little too whimsical to be true but the others assured him she was being honest.) her name is Diona, meaning “spring water,” but everyone calls her Di Di.
•The second girl is 14. She was cared for by sea nymphs in the deepest depths of the ocean, and was named Nerine (neh-Reen) after the naiads. She has choppily cut black hair with strands of green and blue, a side-effect from being at such deep levels in the sea for so many years. Much like Percy, she has a major, if not an even bigger, attitude. She is phenomenal with horses and becomes the best Pegasus rider at camp within days.
•The first boy is seven years old, and looks the most like Percy out of the bunch. He was raised near Poseidon’s court by a group of protecting (but not all that smart) mermen. He loves to run around camp causing trouble and making little wave explosions with whatever water is around and going “BWOOSH!” His name is Hurley, meaning “sea tide”, but everyone calls him “Little Percy”, or “Percy Jr.”
•Little Percy flirts with Annabeth.
•The second boy is 9 years old, and was also raised by sea nymphs, but ones that lived in shallow waters and quiet bays. He took after the nature of the nymphs who raised him, calm and peaceful, kind and loving. He loves to sit in the waves of Long-Island Sound and stare at the water wistfully. He was named Fen, which means “Marsh”, and also, “Peace”. Like his sister Nerine, he is very good with horses and pegasi.
•The last boy is 15, and was raised by the Ichthyocentaurs (Fish centaurs). Because of this, he knows a lot of handy talents and has been somewhat trained to fight. He is very friendly, but often gets a brooding, deep in thought expression when he thinks no one is watching him. His name is Beck, meaning “Stream.”
•Percy and his siblings all fall asleep in one another’s embrace to the sound of the ocean waves crashing against the rocky shore outside
•The next day, Percy springs out of bed and sprints outside, running all around camp yelling “I have siblings! I have siblings!” And everyone rolls their eyes like “we know stupid”