Gelpess is a Lost Number Zoanoid from the Guyver manga and OVA. Since Lost Numbers are experimental prototypes, there is only one Gelpess to have existed. Its zoaform is bulbous and grotesque, but its combat abilities are limited. It possesses the ability to mimic the form of any human being, which it can use to trick enemies into letting their guard down and then will transform and kill them.
After the destruction of their Japanese branch, Cronos abducted Sho Fukamachi's (Guyver-1) father Fumio and his girlfriend Mizuki Segawa. Sho was called out by a group of Cronos troops who used Mizuki as bait for a trap to capture the Guyver. Sho co-operated and he and Mizuki were handcuffed together, meaning if Sho tried to bio-boost, the energy field he creates on transformation would vaporise Mizuki. Sho's soon-to-be ally Masaki Murakami came to the rescue but the Cronos troopers zoaformed and attacked him. As Murakami attempted to lure the Zoanoids away, he tried to warn Sho that Mizuki may not be who she appeared to be. A wary Sho tricked "Mizuki" with a question about her birthday. "Mizuki" blew it and so transformed into Gelpess and attempted to bite Sho's head off. Sho transformed into the Guyver and the shockwave he produced obliterated Gelpess.