(and a billion years later I decided to answer on your question uwu)
Pup’s mostly guessing what Frisk meant. His body isn’t made for talking, he just physically can’t do it. Snaky can hiss, growl and purr. Pup just interpret it due to situation. Later Frisk can pronounce some words(usually just names), especially with letter “s”(and he adds “s” if it’s not in the word xD) but it’s still difficult for him. He actually can use telepathy to talk… snek just thinks he has nothing to say >w> In any case, Pup is talking really much, and Frisk likes to listen to him. That works really good 😀
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