Hecate the Witch, whom we met previously shape changed into a cat.
Like, I figured that out when the cat first appeared; after all, we'd just been told that Hecate would soon be arriving, and suddenly there's this cat with powerful magic keeping people from knowing about the confrontation between Our Hero and She Who Guards Against Evildoers, and then making friends with Our Hero.
We'll see how quickly Routa catches on to this, that Hecate is the cat.
Once he does catch on, it will be interesting to see if he realizes that he now has an intermediary who can carry messages back and forth between himself and the humans.
It will be interesting to see if she understands Beastspeach while in human form. That would alleviate the need for her to shift back and forth between cat and human form to convey messages.
Oh, as to how he's been doing as well as he has communicating with the humans?
Remember, he's a reincarnated human, so he understands where they are coming from, and thus has a much better idea how to model his behaviour to convey his feelings to them than your run of the mill pet does. And given how well some of the pets I've known have done at conveying meaning without that knowledge...
I don't know how he would go about it, but he needs to find out if they have something akin to the game of Charades, and if so, learn the rules. Once Hecate convinces them of just how intelligent he is, using their version of charades when she is not available would allow him to carry on a much more meaningful conversation than previously.