...Do spirit beasts contract with humans in his world? It doesn't seem totally unbelievable, at least...doesn't seem like there's much point interrogating her about it anyway, considering she's not the one who originally...contracted it?

Meh.

"So you're a sort of beast tamer, and--" here he turns to Buttercup again, "you said you fight often? What do you do?" How much he believes in her fighting ability is something we'll put aside for now, but he is genuinely curious as to what exactly she does in all those supposedly fight-filled missions she mentioned earlier.