OVA released with the 9th volume of the manga.

Each of the six volumes of the Maken-Ki! Blu-rays will carry a 10 minute long short anime "Doki Doki! Maken-ki! Himitsu no Kunren". Secret trainings with the main characters will be animated.

What do an alien, former ninja, weak telepath, and boy pretending to be a girl have in common? They are all classmates at Kashihara Girls' Academy, and they hide their true identities from the world. Rikka, Chiyo, Sekine, and Tsubasa seem like the best of friends, but they each harbor a monumental secret that they cannot reveal, no matter what. In order to continue masquerading as normal middle school girls, each of them must invent creative ways to conceal their true selves. But that is easier said than done, as the four students end up entangled in numerous shenanigans along the way.

Drift peacefully into Neo Venezia, a city on the planet Aqua (formerly known as Mars). By the 24th century, humans have found a way to colonize the previously uninhabitable planet. As futuristic as that sounds, Neo Venezia is still teeming with rustic beauty; gondolas on wide canals and waterways are the main mode of transportation. The city itself is a faithful replication of Manhome's (the planet formerly known as Earth) Venice. To make sure that residents and tourists alike get the most from Neo Venezia's many wonders, companies offering guided tours via gondola were formed, one of which is named Aria Company. This is the workplace of Akari Mizunashi, a free spirited teenager from Manhome who is now a novice Undine (the title given to tour guides). Join Akari as she becomes intimately acquainted with other Undine, tourists, Neo Venezia's residents, and even the city itself, learning many valuable life lessons along the way, such as the wonderful truth that there are such things as manmade miracles.