Tony
Steve was supposed to be testing the kid first, but Tony wanted to keep working on improvements for the the next incarnation of his Iron Man suit. So he went in first to test the kid's intelligence, but he really didn't want to babysit an amateur. Especially a surly teenaged boy who didn't understand what was really at stake. He walked into the interrogation room where Percy was waiting.
"What do you know?" He asked. Percy just raised an eyebrow.
"Know about what?" He snarked back. Tony grumbled as his brain whirred quickly through different questions.
"Fine, what is... heroic, in... latin?" Tony asked randomly. Percy snorted and shifted.
"Heroico metro, or heroicus or herous or epicus." The kid answered without missing a beat. Tony frowned as he quickly googled the answer. To his surprise, the kid was right and he'd even pronounced it correctly. Tony's mouth twitched.
"You know latin?" He asked, impressed despite himself.
If Tony was honest with himself, he didn't actually dislike the kid. He just didn't want anybody new to come in and disrupt the routine of the team. Change wasn't something he liked; unless he was the one creating the changes. Then change was fun. The kid nodded shortly.
"Ok, that's good enough for me." He said and walked out. The other Avengers looked at him agast.
"What? The kid knows a dead language. He's smart enough."