Noctis lunged forward, so he could try and give his supposedly best friend a noogie, because, "You're the worst friend. What about the bro code! What about keeping secrets!"

Of course, Noctis trusted Prompto with his life. He knew that there would be no real blackmail, that Prompto would take any secret of his to the grave. But he also knew that Prompto was being a giant brat and deserved to get his hair completely ruined.