Derived from the wiki:

Pebble acts like a dog, despite being a pet rock. He is affectionate and playful towards most Toon. He also seems to show concern for Dandy, his owner. He cannot speak English but seems to be able to comprehend it. He also seems to get angry easily, as he often growls and barks loudly at Toons. However, dialogues with Pebble suggest his barks function as a substitute for words and are able to convey meaning. Pebble seems to have some kind of eeriness to him, given his staring problem and tendency to weird or creep others out. This could also be due to his connection with Dandy, as he would be the one to be with him more frequently than the other Toons. Similarly to real dogs, Pebble also enjoys playing fetch as seen in an interaction with Vee, along with treats and is possibly being overfed at times by other Toons.

This is a PET TOON! I'm unsure if I WILL choose to cast people for the roles of Pebble and Coal, but i'm choosing to give it a chance anyways as it's funny to me lol

No particular sounding voice in mind. Simply a pup.