Rentaro is voiced by Marc Biagi.
In 2010, Nancy first meets Rentaro as she attempts to enter the baths at the Ryokan Hiei. He explains that when the baths are draped with a blue banner, it's for men only, but when the banner is red, it means women only. Hitching up his toolbox, he promptly stumbles and trips out the door.
After this initial encounter, Rentaro can be found in his little tool shed out in the ryokan gardens. Though slightly clumsy and awkward, he has an oddly charming sweetness about him and enthusiastically gives Nancy a booklet full of number puzzles. Inspired by the robotic cat (Suki) he gave to his girlfriend Miwako, he is currently working on a robotic dog called the "Rentaro 4000" to give to her as well. He has always loved the city, and used to sneak out with Yumi to take trains around the city when they were children. He still enjoys going to the city and visits Yumi's apartment every chance he can get, dreaming of one day owning a city apartment himself. He has been trying in vain to convince Miwako to move to the city with him, something she considers "selfish", though he insists that it's for her own good too. He's convinced that there has been too much fighting and sadness at the ryokan for her to be truly happy there, especially since her mother died. As for the reputation the ryokan has for being haunted, he doesn't really believe it, although he admits some of the things that have happened have been quite spooky.
As Nancy explores the ryokan, she eventually discovers a secret passageway to a hidden, run-down bath. After she completes the gigantic nonogram puzzle, she is attacked by the yurei and is shoved underwater, nearly drowning before she manages to escape. As she examines the yurei's body closely, Nancy discovers that the creepy thing is a robot and recognizes the mechanical part Rentaro had made for the robotic dog in the yurei's arm. Realizing that he must be behind the hauntings, Nancy tracks him down.
She eventually finds him rigging the doors in room 33. As Nancy confronts him, he reveals that he faked all the hauntings to try and force Miwako to either leave or close down the ryokan, as he truly thinks it's not a good environment for her. What with the constant fighting with her sister, being perpetually reminded that she is not supposed to run the ryokan even though she loves to do so, and finally, her mother's death in the baths, why would she want to stay? In Rentaro's mind, the ryokan holds too much pain for his girlfriend.
Even so, he certainly did not help ease that pain and did almost drown Nancy. Having secretly recorded everything he's said, Nancy can choose to either give the evidence to the Shimizu family or allow Rentaro to confess to Miwako himself. He either leaves and is never seen again, or apologizes for many months until a furious Miwako permits him to work at the ryokan and undo the damage he has done to its reputation. Miwako understandably broke up with him when he confessed, but he is hopeful that she may eventually come around. Nancy thinks he shouldn't hold his breath.
- Renograms are his favorite type of number puzzle.
- He has always wanted to visit New York City.
- "It's just another example of how awesome numbers are! Letters are cool and all, but they just make words. I mean, get with the game, letters!"
- "You know it. You love it. The original, sudoku!"
- "Some people like adult contemporary. Don't be one of those people."
- "I don't know why we don't mention this at the front desk, this is useful information."
- "Ghosts are just what happen when guilty people let their imaginations run wild."
- "Considering everything that's happened here, why would anyone want to stay?"
- "A ghost doesn't need to be real to haunt you."
- "Believing in something, that makes it real to you."
- "And this is why I work with machines. They're totally oblivious to akwardness."