[Ah, he's stuttering. It takes a moment, but he gets "I hope I'm" and "interrupting anything" and that's good enough context for him. He raises his hands in front of him, waving them a little dismissively with an equine laugh.]

Of course not! How can you interrupt a slumber party that you were invited to? [That he conveniently chose not to tell you about because surprises are way better.] I wanted to bring my friend over too, so I thought "why not turn it into a party?" We got snacks and games and everything!

[How can you say no to that? He gestures to the boy next to him.]

Plus, you wanted to meet my friend, right? This is Saihara Shuichi-chan! Saihara-chan, this is my roommate Aziraphale-chan!