Spawny is a Rabbid that appears in Mario + Rabbids Kingdom Battle. In the Mushroom Kingdom, he is differentiated from other Rabbids by his white muzzle, blue belly, and ears, the fused SupaMerge headset that covers his eyes, and the on/off button on his lower back. His ability is to fuse other beings with Rabbids, doing so to create the Pirabbid Plant; however, said power is uncontrollable, and activates whenever he's stressed.
His name is given to him by Bowser Jr., who seeks to use his powers to take over the Mushroom Kingdom. Although he is a major catalyst for the events of the game and forms an alliance with Bowser Jr., Spawny is not evil.
Spawny starts out as an ordinary Rabbid, but when he and many other Rabbids are transported to the Genius Girl's laboratory, Spawny steals the SupaMerge and uses it to fuse other Rabbids with objects, doing so to create Rabbid Peach and Rabbid Mario. When Spawny enters the Mushroom Kingdom, the SupaMerge ends up meshing with him, turning him into what he looks like throughout the game.
Beep-0 informs Mario that the SupaMerge had been corrupted upon entering the Mushroom Kingdom, and he agrees to find Spawny. However, just as Mario and co. are about to catch Spawny on a tower overlooking Ancient Gardens, Bowser Jr. appears and befriends Spawny. Mario and co. traverse the other three worlds and eventually defeat Bowser Jr. in his mech, Mecha Jr., retrieving Spawny in the process. However, the portal over the Mushroom Kingdom releases a dragon named the Megabug, which captures Spawny and harnesses his power to unleash more beings. Mario and co. end up defeating the Megabug and freeing Spawny and Bowser (who had also been corrupted by the Megabug). Spawny is then seen in the pre-credits scene, where he admires the statue erected in honor of Rabbid Peach.
When he is still a regular Rabbid, Spawny is mischievous, curious, and playful like his fellow Rabbids, while always causing trouble without thinking about the consequences. After his transformation, Spawny becomes timid, naïve, and childlike. This causes Bowser Jr. to take advantage of him, bullying him into using his abilities. Because his power comes from fear and stress, Spawny has little to no control of his abilities.
Bowser Jr.: Upon seeing Spawny's abilities, Jr. decides to use Spawny for his personal gain in order to defeat Mario and co, and make his father proud. After giving him his name, Jr. preys on Spawny's naïve personality in order to befriend him. The little Rabbid shortly learns that Bowser Jr. is just using him and makes several attempts to get away from him. Despite all that, Bowser Jr. generally does care about Spawny, showing concern when he falls out of his clown car and even scolding at Mario and co. when he is kidnapped by the Megabug. After Spawny is safe, thanks to Mario and co., Jr gives him to the heroes, relieved that the young Rabbid is safe.
Mario, Luigi, Princess Peach, and Yoshi: Due to being overwhelmed by his power, Spawny is uneasy around Mario and friends, fearing that they are his enemies. Spawny is startled when he and Luigi wake a Piranha Plant, and it scares him, causing him to create the Pirabbid Plant. Throughout the game Spawny grows uneasy around the heroes causing more stress, but eventually comes to realize they want to help. After the battle with Mecha Jr. Spawny falls out of the clown car and is saved by Yoshi. After being rescued from the Megabug, Spawny eventually befriends the group and settles down. He seems to be the closest with Princess Peach as his trophy shows her hugging him while he smiles, and she even holds him during the unveiling of the Rabbid Peach statue. In an artwork of them, Spawny is giving the princess a holographic mustache much to her surprise.
Rabbid Mario, Rabbid Luigi, Rabbid Peach, Rabbid Yoshi, and Rabbids: Before his transformation, Spawny treats his fellow Rabbids with compassion and mischief. After his transformation, he becomes timid and shy around them, possibly from the stress of his new powers, trying to avoid them so he won't hurt them. With the heroic Rabbids, he is afraid of them because he thinks they are his enemies, but eventually warms up to them seeing them as his friends.