Fracktail is the robotic guardian of the 2nd (Orange) Pure Heart, he has been waiting 1,500 years to let the hero pass. Fracktail also acts as the final boss of Chapter 1 due to Dimentio shocking him. His voice should sound robotic, but also should sound natural, so I would aim for 50/50. Fracktail, since he's a boss at the end of a chapter, doesn't have the biggest role so there is not much to say about him. Along with a robotic voice when he emerges his voice should be booming, but then settle down a tiny bit after he starts to scan SpongeBob.