Percy held the sky up so Artemis could be freed in order to fight Atlas in battle and protect the others...

it hurts.. I feel like it's crushing my very soul. That's exactly how Percy felt.

If only... if only someone would help him carry this burden.. if only someone.. Annabeth.. where.. are.. you?

He's been holding up the sky for at least 3 minutes.. it was not that long.. but it hurts.. His brain was fogged, but he heard distant yelling and the clashing of swords.. until someone came up next to him, yelling at him to let go of the sky, or if not, she'll carry the sky alongside him anyway..

Annab–no... she was blonde.. this girl beside him, who lifted her arms, carrying the sky alongside him, wasn't blonde.

She was.. looking up the sky.. talking as if commanding the sky to soar high back to where it belongs.

He was getting weaker and eventually collapsed.. all he knew was that this girl wasn't an ordinary demigod.. she commanded the sky, and it complied. But the battle wasn't over, still.