I love the hc that Percy has his own version of charmspeak ("Siren Song") but it's more dangerous bc it's not as natural.
So my own take:
The first time he uses it, is by accident on the Argo II. They're being swarmed by monsters and Percy's main weapon has always been his tongue, manipulating and goading his enemies, so he panics and just yells 'Stop!'
They do, if only for a few seconds, and the air that left his lungs now can't return, he can't breathe-
Piper has to order him to do so.
He decides not to do it again, unless they have no other choice. He does do it, once more. He's smarter about his order this time. The monster is on the verge of falling under Piper's spell, and the others are too busy with other enemies, so he commands 'Listen to her'.
He cannot breathe again, but Piper manages to help him much faster this time. His voice is raspy afterwards, but heals after a few days.
And then he and Annabeth fall into Tartarus.
There is no Piper to force him to breathe here, but he's dying anyway from the arai, might as well try to protect her.
'Leave her alone! Remove her curse!'
For a moment he suffocates, and he hopes the curses kill him soon as the burning in his lungs is unbearable. Suddenly, the burning seems more real, he coughs out firewater.
The Phlegethon healed him.
He practices in Tartarus, their bodies injured and exhausted enough that, more often than not, fighting isn't really an option.
He gets used to suffocating. After a while though, it stops, as he learns to withstand in. It's not nearly as powerful as Piper's. It's enough to survive.
When they get out, him and Piper start working together more often. Because while you're focused of resisting the pretty voice with a pretty face, there is someone sneaking behind you, just as deadly, and suddenly the pretty voice belongs to whom you trust.
