$100 on Robin Arryn. He's going down!

Thanks! According to some other sites, he was in earlier episodes, but I am blanking on them too. Anything significant you remember?

Can anyone remind me the backstory on Catalan, and why it's such a big revelation that he and MacLeish know each other? I've been watching the show from the beginning, so no spoiler warning needed; I'm just completely blanking on why this is critical.