Okay, well, on the one hand that's frightening and he's really not sure he wants to let whatever that is into his room.

On the other hand... It's probably not random chance that two weird things happen to him in the same day, and this might help explain whatever it is.

He goes to his bookshelf, grabs a heavy book, then returns to the window, quickly opens it, and backs away.

"Are you deaf? I've been trying to get you to open this forever! What were you even doing?" the stuffed toy complains with a high childish voice.

"I was listening to music," responds Theo in a somewhat hostile voice, eyeing the creature warily. "What are you doing? What are you, and why are you here?"

"I am Cerberus, guardian of the Clow Cards, and you have one!" He stops and holds his head between his paws. "Ooowww it hurts!" he says as he floats in—the wings are apparently merely decorative. "Look what you made me do!"

"... What I made you do? How is you hitting your head against my window my fault? ... And what are the Clow Cards?" (He's pretty sure he can guess, but just in case this is, for some reason, a totally unrelated weird occurrence...)

Cerberus starts zipping around very quickly, looking for it. "I know you have it! Windy! And where are the others?"

"Windy? 'The others'? Are those the 'Clow Cards'?"

He floats over toward Theo. "You really don't know?"

"Know what? Because I know lots of things, but what the 'Clow Cards' are isn't one of them." (Or maybe it is, but he's not willing to give information without getting some first.)

He slowly moves over to his bed for no obvious reason.

He sighs and sits on the bed. "I woke up in this museum and the Book was open and all the Clow Cards were gone. I tried to find them and could only sense Windy but it's not here..."

"You woke up in a museum? What were you doing before that, and what do you mean you could 'sense Windy'?"

He stops before he gets to the bed, still maintaining some distance from Cerberus. Still somewhat wary, he hasn't put the book down, but fortunately it doesn't seem he'll need to attack.

"...I was actually supposed to be guarding the cards but I... fell asleep."

"... Oh." – that's not really what he was expecting. "So why'd you come here?"

He's starting to feel a little sympathetic towards this creature.

"I told you, I was looking for the cards! If the Clow Cards aren't all collected and sealed again, a great catastrophe will befall the world!"

"A great catastrophe? Can you be more specific? And why were you looking here for the cards? Did you 'sense Windy' and come here?"

Yeah, perhaps "hostile and withholding information" wasn't the best way to go about this. In fairness: weird creature, at night, after a weird day.

"I—" His ears twitch, and he looks around.

Theo looks suspiciously at the bear, then looks around as well. "What?"

"I am still sensing Windy here. It's not usually this quiet."

"Quiet? Didn't you say it was a card...? Or is it... special; like, have I mentioned that you're a winged, talking bear? You do know that, right?"

Cerberus inches his way in the general direction of Theo's pillow.

Theo runs over to the pillow, attempting to block Cerberus from reaching it.

"You have it!" Cerberus accuses.

He grabs hold of the card, stands up, and holds the book at the ready again. "Yeah, I do, but how do I know you're who you say you are? Even if there is a 'Guardian of the Clow Cards', how do I know you're them?"

"You were the one who scattered them! Why should I trust you to have it?"

"I didn't scatter them, I stood near a book and then touched a card! How should I know that'd scatter some magical cards? Magical cards don't exist!"