Sgt. Frog is a Keronian (That means he's from the planet Keron) who has tried 9000 times to invade Pekopon but instead took a break and stayed here like Orbulon.
Sgt. Frog appears as a boss. He starts the fight by flying around in his scooter thing, trying to shoot you with lazers from the Kero Ball. In this state, you must hit him 6 times. Afterward, he summons his Gunpla Army. In this state, you must defeat all Gunpla and then hit him. Do this 6 times and he'll run away.
Sgt. Frog appears as a boss here as Sgt. K, but this time, he has help from Sgt. R. First, you must attack Sgt. R with Link's Sword 8 times. Afterward, Sgt. K attacks you with the K-Gunpla suit which will shoot you with the Kero Ball again. You must attack the head 4 times and after the suit falls, take the weapon and continue to attack the head. After you defeat them, they promise to help you find Malleo.
Sgt. Frog appears as the final boss. How to defeat him, not even Fantendo knows, and they created the game in the first place. But after you defeat him, he uses "Reveal" on you, thus realizing that 9-Volt is a Keronian as well.