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Shinnosuke has dark hair (brown in the OVA) with a fringe and a small ponytail. He wears a bandage around his head and a blue shirt under a dark blue overcoat, which is similar to what his grandfather wore as a zookeeper.
Shinnosuke lacks tact, which can make appear rude due to his blunt honesty. One of his major flaws is that he has no short-term or long-term memory. He often forgets names and people altogether and often gets caught in his own traps because he forgets where he placed them. One of the few things he is able to remember on a regular basis is Akane Tendo's name, which is a testament to his deep feelings for her. In the OVA this frequently earns him a Megaton Punch from his grandfather, who he tended to forget often.
Many years ago, Shinnosuke's grandfather ran a small petting zoo in Ryugenzawa, and when the animals escaped they grew to enormous sizes from drinking the Water of Life in the forest. Shinnosuke was raised by his grandfather since he was a young boy and no mention is made of his parents. The pair live in a house deep in Ryugenzawa, where they protect the local villagers from the giant animals.
When she was young Akane was vacationing in Ryugenzawa with her family but got separated from them. She was about to be attacked by a giant platypus, but was saved by Shinnosuke, who sustained severe injuries when the platypus inflicted deep cuts on his back. To keep her safe, he gave her the Horn Whistle. Shinnosuke's wounds never fully healed and he became dependent on the Water of Life to provide a steady cure, such that without frequent exposure to it he will eventually die. Because of his grandfather's age Shinnosuke is now largely responsible for the giant animals.
Years later Akane recalled her experiences in Ryugenzawa when she heard a news report about a monster from there. She found the Horn Whistle among her things and suddenly left to seek out Shinnosuke. The two were reunited in similar circumstances when he again saved her from an attacking giant platypus. However soon after Akane's arrival the Orochi began to awaken from hibernation and block up the main spring supplying the Water of Life on the surface. The animals reverted to their normal sizes and Shinnosuke began to experience withdrawal symptoms in the form of sudden fainting spells. At first Akane and his grandfather tried to keep the possibility of Shinnosuke's possible death from him, but he found out and accepted it calmly. Ranma and Ryoga decided to help after learning about the situation as well.
After Akane got the moss from the main Orochi head, Shinnosuke's grandfather scrubbed it into his wounds. The wounds permanently healed, allowing Shinnosuke to continue to safeguard Ryugenzawa. However he forgot about his prior fight with Ranma as well as his confession to Akane.
Like many of the other male characters in the series, Shinnosuke is in love with Akane Tendo, and like Ryoga is willing to eat her cooking even after the Water of Life began to run out and its quality returned to its usual state of being bad. He confessed his feeling to Akane but afterwards forgot that he had done so and wished that he had been able to tell her how he felt when he left.
His relationship with his grandpa is he raised Shinnosuke and fears the day he will die.
Like his grandfather, Shinnosuke uses a push broom as a weapon. He is fast enough to dodge and counter attacks from the giant animals and during his and Ranma's brief fight he was able to nick Ranma. Until his wounds were permanently cured his reliance on the Water of Life to stay alive was his greatest weakness, as he would suffer withdrawal symptoms once he stopped drinking the water.