Koisen's body resembles a cross between a koi fish and an eel. He is appropriately four feet long and has a sickly-green color scheme for his scales and fins. He has two short pincers, which he uses to help himself swim and gather food and many black wiry appendages down his body. On each side sits two very large appendages, meant to resemble hands, giving him a more intimating appearance to ward off predators.
The Omnitrix symbol is located on his forehead.
- Sharp Quill Appendages- While the black-and-red appendages on his body resemble arms and legs, they are not actual limbs, but very sharp, flexible quills akin to cheese-wire, that Koisen maneuvers by fast and agile swimming.
- Corrosive Venom- The intense sharpness of Koisen's quills can allow him to cut deep into predators very easily, which then releases an extremely potent venom into their body. The venom paralyzes the victim within seconds, then slowly rots them from the inside.
- Underwater Breathing- Koisen’s gills allow him to breathe underwater.
- Frail Build- Koisen is mostly protected by his assortment of quills, but his body is very frail, should a predator land a fair hit.
- Aquatic Maneuverability- Koisen also needs to be submerged in water to move and attack. Although the Omnitrix gives him life-support out of the water, he still needs to gain momentum by swimming to correctly maneuver his quills.
Before 5 Years Later
- The Omnitrix collected Koisen's DNA at some point before the Battle in the Null Void.
5 Years Later
- In Chapter 7: Fold, Kevin absorbs Koisen's (or a Shishiana's) DNA on his left arm for the upcoming battle with The Alliance in the Keshawn Desert.
- In Chapter 8: Risen, Kevin utilizes Koisen's DNA in a battle against the Alternative Ben Tennyson's.
- Koisen is the first 5YL transformation to debut by someone other than Ben. His DNA sample was used by Kevin in Chapter 7: Fold.