Multeye is a Nighlok who managed to steal a mysterious weapon called Advent Gun from Ventara (although its unexplained how he got it & how he got into Ventara) & did not need the Sanzu River water. He becomes a "Nighlok Rider", this universe's (the Power Rangers universe) first ever Kamen Rider, in a world that was supposed to be without riders.
With all the crevices in the Kamen Rider-like helmet & armor, he can easily summon Mooger footsoldiers. He also can summon Contract Beasts through his Attack Vent Cards. Kamen Rider Dragon Knight & Siren team up with the Samurai Rangers to destroy this monster. He did not grow giant.