Percy
He was furious with himself. How could he have never noticed the Mark of Athena symbol carved into the marble? How could he have never noticed that by pressing on the Mark, it triggered a mechanism so that the whole gods be damned headstone moves out of the way to reveal a set of stairs underground? The thing glowed for Zeus's sake! Steve noticed it from clear across the row! Percy was more than just angry with himself. He was furious. How could he have never realized that Annabeth wasn't dead in the first place? He had known even after she fell over the cliff when they were just kids!
Basically, Percy was furious with himself. He waited, every muscle in his body tense as the rest of the Avengers gathered around him to hear the plan.
"We go in, find Annabeth, and get out. If we run into any monsters let me or Thor kill them. You won't be able to without godly metal and we don't have time to get you any. Protect Annabeth, that's your primary goal." Percy gave each of them hard looks before relaxing his glare. They didn't deserve his anger, not when they were about to follow him to fight monsters they can't fight, to save someone they've never met.
"Don't die. You can't help anyone if you're dead." Percy watched each of his new friends nod before he turned and charged down the stairs.