Yeah, sure. Frisk and their sibling are always fine, too. They stare at him evenly for a beat or two, but there's nothing else in their body language that suggests they don't believe him. Rather -- after a moment of silence to mull over his words -- Frisk simply begins mildly kicking their boots again, leaning forward over the pillar with a casual sort of slouch.

They won't call you out if you don't do the same, my guy.

"Have you ever heard of Shadows?"

Capital-S Shadows, not that they make that distinction clear, verbally. Something else they and their Partner encountered in the castle. Which should be off-limits for discussion, Frisk and Chara having agreed to keep everything about their previous lives under wraps, but Frisk figures that Taako falls within the same safe category as Lup does. They're a bit fast and loose with their own rules that way.