While Rantaro was aware of the fact that Maki was the Ultimate Assassin, he was determined to stop her from killing Kokichi. He wasn't entirely sure what he would do, but he was going to do something to stop her. No matter what it would take, as long as Kokichi was safe, he would be happy. However, he was concerned about Kaito as well, wondering if the assassin would target him as well. He hoped not, but either way, he would protect his lovers.
"I'm not sure just yet, but I will find a way to," he assured Kaito. "You were friends with Maki before this. If there's any information that I should have, please let me know. I will protect you guys somehow. That's a promise."