Awkward as the silence is, the Batman does relax in it enough to flop back onto his butt and straighten his spine, settling into a more human sitting position instead of crouching on all fours. Frisk doesn't seem like they're on the verge of running away from the big scary bat-monster, after all, so perhaps there's no need to try and make himself look smaller.

"Frisk," he repeats. There's the slightest of hesitations before he continues, "How long have you been here?"