Yonezawa-kun, a green-skinned and sturdy kappa, inhabits a small forest lake near Shinobi Gakuen. Though naturally polite, friendly, and deeply peaceful, wasteful treatment of nature ignites intense anger within him. His profound connection to the environment manifests in frequent wise sayings urging others towards harmony.

He shares a strong, affectionate mutual romance with Yusura, an animal-themed kunoichi-in-training. Receiving gifts from her, especially cucumbers, brings him particular happiness. These cucumbers hold deep personal significance, making their theft or rejection a rare source of irritation that disrupts his usual calm.

A pivotal moment occurs when a cucumber gifted by Yusura shields him from a fatal blow while he was under mind control. This intervention breaks the brainwashing, demonstrating the cucumber's symbolic importance to his well-being. Yonezawa-kun primarily supports Yusura and occasionally aids the main characters with his natural wisdom.