Heikinteki Josei is the first boss encountered in this game. She can be found on level 5. She has with a pink lunchbox with a cartoon Genbu on it. When you fight her and win, you get her lunch box and 5 karma.
The next boss appears at level 15, a snake-wrapped tortoise named Genbu. It also appears in one of the 'assist ally' minigames. If a player defeats Genbu, he/she rewarded with a relic, called Genbu's Scale (+8% attack speed, +10% critical), gold, experience, and an achievement worth 10 Karma. Also, if the player defeats it using only food items (Fried Shrimp, Negi, Stinky Fish and Pocky Stick) as weapons, he/she is given another achievement worth 10 Karma.
The third boss appears at level 25, called Tondeiru Josei, who is Heikinteki Josei's sister. If a player defeats her, he/she receives the Daikatana (20.57 DPS / Slow Speed), gold, experience, and an achievement worth 10 Karma. If he/she defeats her while a ninja holding the Daikatana is still alive and attacks her, the player gets another achieivement worth 10 Karma.
The last boss appears at level 35. It is called Mecha-Genbu. If a player defeats it, he/she receives a Laser Visor, (47DPS), a larger amount of gold and experience, and an achievement worth 10 Karma. If the player defeats him after he launches his nuke, but before it lands, he/she earns the stamp "Nuclear Arms Race," worth 10 Karma. If he/she defeats him after the nuke lands and survives it, the said player receives the achievement "We Had a Falling Out," worth 10 Karma. The nuke takes ~2 rounds detonate on your ninjas. Defeating the Mecha-Genbu requires a high amount of health, power, armor, and weapons.