Minun first appeared as an Egg that was owned by Kaisei. After being spotted by the International Police, Kaisei gave the Egg to Hareta as a gift and ran off to escape his pursuers. Hareta kept onto the Egg and nearly caused it to break several times with his clumsiness.
When Hareta and his friends were battling Heatran on Stark Mountain, the Egg hatched into a Minun. Despite the danger they were in, Mitsumi and Mars were distracted by its cuteness, causing Hareta to try the same on Heatran but only getting attacked by it in the process. In their attempt to calm Heatran by putting the Magma Stone back in place, the stone was tossed by Mitsumi to Koya to Minun and then to Hareta who kicked the stone back into its spot, calming Heatran down.
Minun and Hareta
In Canalave City, Minun was used to fight Saturn but was easily defeated by his Magmortar and ran away. When Saturn had Magmortar attack Minun, Hareta's Empoleon stepped in to protect it from the flames. Annoyed at Minun's cheering Empoleon on, Saturn ran up and grabbed Minun. Minun responded by shocking Saturn and paralyzing him, allowing Hareta to defeat Saturn's Pokémon and win the battle.
Minun cheering Empoleon on
When Hareta and Koya fought Charon and his mind-controlled Pokémon, Minun used its Sing to put the enraged Pokémon to sleep without hurting them in the process.
Minun was seen in Hareta and Koya's final battle where it cheered on Empoleon as it charged to fight. It was last seen with Hareta in the ending of Pokémon Diamond and Pearl Adventure! where it joined Hareta, Mitsumi, and Koya as they set off on their next adventure.
Personality and characteristics
Due to it being a baby, Minun isn't very strong in battle and mainly resorts to using its cuteness or using support moves to fight. It has shown that when it needs to be, it can be strong enough to paralyze a human easily.
Minun is very playful and has the tendency to run around, causing trouble in the process. Minun is constantly shown cheering on others with electric pompoms when it isn't used to fight. Despite being born after it, Minun seems to have a connection to Kaisei's Plusle, as shown when Minun was born, Plusle could sense its hatching even though it was in a completely different location.