Post-tartarus Percy and Annabeth and even post tartarus Nico weren't written well at all. Tartarus makes them manic, always on edge, jumpy. This is PTSD and it doesn't just go away. Not to mention, they lost Bob and Damasen.
Yes, sure, one could argue there weren't many opportunities for Rick to explore how Tartarus changed Percy in Heroes of Olympus series. There still should have been mentions.
Rick focused on Bob and Damasen part for a bit and then never again. It's like after Tartarus arc, Percy and Annabeth were back to being themselves, which is such a character assassination. Here are a few ways in which I think Tartarus changed Percy:
PART 1-PERCY
Percy becomes hypervigilant at all times.
He can barely sleep, and whenever he does, he is always affected by terrible nightmares.
He feels extremely guilty about not being able to save Bob and Damasen and hence is now extremely protective of his other friends and family.
He no longer feels at all safe in his cabin or at camp and avoids it since Hera abducting him from there led to the whole thing.
He spends large amounts of time away from the camp and avoids other demigods because he is constantly afraid he is going to hurt someone.
Percy only ever feels truly safe by the sea .
Percy knows how delicate Sally and Paul act around him, and he hates it and hates himself for it. They don't say a word even when his screams wake them up. In fact, they just sit beside him, holding his hand and letting him cry or scream. Mostly, he just finds himself shutting down.
Poseidon also makes his own protective tendencies known, and when Percy gets too manic to stay anywhere, he whisks him away to Atlantis to calm him and give him comfort.
Every night, Percy finds a place to look at the stars and repeat the same message : Bob says hello.
He becomes an entirely different person when attacked when fighting monsters. A killing machine. It scares most people especially Annabeth.
Percy becomes a lot more aloof and likes being by himself (so he doesn't have to bear the responsibility for protecting someone other than him for a while)
He becomes a bit more pensive and talks less than usual.
He hates his own power but doesn't shy away from using it in dangerous situations. But this leaves him with a long internal struggle.
He begins to think of himself as monstrous.
He can't feel at ease without feeling Riptide in his grasp.
Somedays the nightmares, the memories get so worse that he has to make himself bleed to take the worst of it away.
He becomes skeptical of most things: people, and food and objects.
Everyone makes sure not to bring Calypso in the same room as him lest he kill her then and there. It takes him a long time to forgive her.
Percy is now so good at sensing monsters that he is always ten steps ahead of everyone.
The worst of it is Percy's constant need to fight something. He spars with multiple strong opponents to at least make it a bit challenging for him. It makes him feel like Ares, and he hates it.
Percy's first true genuine smile is when he finds out he is going to have a baby sister.
Will cover Annabeth's and then Nico's in the next parts. This is enough ptsd for all of us for one day.