Hunter ;
He should have seen it sooner but granted his ADHD...his attention span was as small as Grover's satyr horns...which were really small.
Once he put the puzzle pieces togheter, it was crystal clear.
Thalia was approached by the Hunters at some point in her life.
It made a lot of sense now that he was thinking about it. Why she hated Zoe, why she was angrier than usual when they lost the game of Capture the Flag.
They had the same kind of toughness, Thalia and the Hunters. There was a resemblance but it could have also been a coincidence so he said, more like a fact than a question. "The Hunters tried to recruit you."
Thalia didn't deny it.
She even admited it that she had refused because of Luke. The thought made Percy both angry and relieved.
Angry because Luke's influence on both Thalia and Annabeth was worrying. He wondered if they would be capable of fighting or even killing Luke if it came down to it.
Relieved because they wouldn't have had this bond if she accepted the offer of becoming a hunter.
Her Choice;
Thalia chose to kick Luke off the cliff, knowing he might die. She chose to resist the temptation of power. She chose to side with the Gods, with them.
For a moment, he doubted her and worried that another supposed friend will turn out to be an enemy.
But that's wasn't the case.
Thalia made the right choice.
However, Percy couldn't say the same thing about the second one. There he was, worrying and nervous and thinking about ways to stop Annabeth from joining Artemis when it was her who he should have looked after.
Thalia was leaving.
The thought didn't sit right with him but he knew there was nothing that he could have told the stubborn daughter of Zeus to make her stay.
They were alike and he never let people change his mind when he took decisions.
So Percy stood, dumb smacked and blushing when Thalia hugged him after she said her pledge. "I'm honoring a friend." she told him.
It sounded too much like a goodbye.
As if that was their last meeting.
His arms tightened around her then, when realization hit him like ton of bricks. Maybe that was going to be the last time they saw each other and it made his chest ache when he thought that they had spend their time togheter more arguing than anything else.
If Thalia noticed his grip on her changed, she didn't say anything. Just called him a hero, saying how proud she was they were friends.
Percy almost didn't let go of her but something made him nod and smile.
Maybe it was the fact that his dad and her dad were in the room and watching them or maybe it was faith.
Faith that Thalia's second choice was just as right as the first one.
Reunion;
Three years passed before he saw her again. The circumstances could have been better but he was glad to see Thalia neverthless.
He was taller than her and older by a few days. If Thalia wouldn't have joined the hunt...she would have been eighteen and older than him.
As such, Percy was sixteen, Thalia frozen at fifteen and the Prophecy was happening. The war against titans, monsters and traitorous demigods was happening.
They hugged briefly before they charged at enemies and fought side by side. They made a pretty good team when their powers weren't directed at one another.
There a few small moments where he took his time to observe her. They were either too tired to keep fighting or standing as Apollo kids healed whatever injury the battle gived them.
Percy saw it all. The way her electrifying blue eyes shone with life, the way she gracefully moved and slashed tons of nasty monsters with her spear, the silver aura that lay around her.
She was amazing but most importantly, she was at peace and Percy felt selfish for wanting her to stay with them, with him after the War ended.
Thalia was better off with the Hunters and he was going to respect that and be happy for her.
He was going to wait and cling to hope that somehow, they will always have their reunions and talks and cheeseburger lunches.