Tesso, or the Iron Rat, is a demon in Japanese mythology said to be the size of a full grown man with metal teeth and claws that could shred almost anything. This demon also had the ability to summon swarms of regular rats to do its bidding.
Tesso was originally a Buddhist monk named Raigo, however, due to being slighted in life he enacted a hunger strike which led to his death. Afterward, the anger he felt transformed his spirit into the demon.
Tesso is the second boss of the SEBEC route and is summoned by Aki upon being confronted in St. Hermelin ???. It appears to be a large mechanical rat outfitted with a large gun. Maki (along with Eriko or Yuka if either were recruited) thinks Tesso looks cute while Masao (and Hidehiko if he was recruited) can't take its appearance seriously (Reiji has nothing to say though). After Tesso attempts to shoot at the protagonist and misses, Kei chastises the others for lowering their guard and underestimating the enemy.