"Look," said Weasley angrily, "we can't trust him, okay. Because – "
"Because I'm a Slytherin, so therefore I'm evil? Honestly? Look, I'm a lot more than just my parents and my house. You barely know me. What gave you the right to judge me based on stereotypes and assumptions?" Leo's voice was mild, but the anger behind it was very sincerely felt.
"Because," spat Weasley, "you're trying to blackmail us!"
"Fair point," Leo admitted. "But still, it's your choice not to trust me. For what it's worth, you can trust me with whatever you choose to."
Potter took a deep breath. "All right," he said, and then told Leo what he already knew: the Chamber had been opened before, and Dobby had warned him about it.
"I see," said Leo. "Is that definitely everything?"
Potter nodded. Leo observed him carefully, but couldn't see any traces of guilt or hesitation on his face.
"And what," asked Marisa, "made you suspect that Draco was Slytherin's heir?"
"You mean other than the fact that he hates Muggle-borns and that he's an idiotic blood-purist?" asked Weasley sarcastically.
"Yes," said Leo. "That description could apply to quite a few people in Slytherin, who's to say it isn't one of them?"
Weasley stared at Leo as if he had never realised this fairly obvious fact before.
"By the way, I can be ninety-nine percent certain that Draco is not the Heir of Slytherin."
"Right!" spat Weasley. "Because you are!"
"No, I'm not," replied Leo wearily. "And I can prove it. My family's bloodline can be traced back through the last thousand years, every last relative accounted for, and none of them are in any way descended from Salazar Slytherin. We can go and look through the geanology books if you don't believe me. Draco does know something, but I don't think it's any more than we know now."
There was a slight pause. Leo wondered if there was anything else that they could ask. It was Marisa who came up with an idea.
"I want some of this Polyjuice Potion," she said. "Just in case I happen to need it for anything."
This time they did hesitate noticeably. "Ron," said Potter, in an undertone evidently meant to be inaudible to Leo and Marisa, "what exactly are the consequences of illegally brewing and using regulated potions?"
"Um… I dunno exactly… I guess we'd be fined and get warning letters and they'd tell our parents…"
"More importantly," said Leo, "you would both have criminal records, and if I wanted to, I could make sure everyone knew about it so your reputations would be permanently damaged."
He stared at the pair of Gryffindors unflinchingly, watching as they mentally weighed up the disadvantages of this against the disadvantages of handing a vial of Polyjuice Potion to someone as sneaky as Leo or Marisa who would be able to do thousands of possibly-illegal things with it if they wanted.
Finally, Potter asked "How much do you want?"
"One vial each," said Leo, ignoring the look Marisa, who would rather have asked for more, shot him.
"We – uh – haven't got it with us," said Potter. "We need to go and get it."
"Okay," replied Leo. "We'll wait here."
Potter and Weasley walked quickly off, leaving Leo and Marisa alone and free to talk.