Gengetsu was a man of both tall and slender stature. His face was accentuated by his relatively long, blond hair, which was worn in a combed-back style. He also had a triangular goatee, a thin moustache, no eyebrows, and very prominent lines at the sides of his mouth. Due to the effects of the Summoning: Impure World Reincarnation technique, he had black sclera but retained his blue irises.
Gengetsu's typical attire consisted of a long sleeved, high-collared, grey kimono, which has the pin-striped pattern that is common amongst the shinobi of the Hidden Mist Village. This article is worn over a dark-green suit, with a similarly colored sash wrapped around his waist, and a pair of sandals which reached up to his calves.
Gengetsu is typically laid back, jovial, and has a somewhat carefree attitude at times. He is also very charismatic and not averse to praising other people while at the same time a bit of a bully, as seen with his unmistakable excitement upon observing the enormity of Gaara's sand attack towering towards him and admitting he often picked on Ōnoki when the latter was young. Complementing this, Gengetsu will take an immediate liking to any shinobi who shows potential regardless of their village of origin or age. Also stemming from this trait, Gengetsu is very forgetful of even major events in his life, such as when he forgot he and Mū killed each other and needed Mu to jump start his memory before remembering their deaths upon being reincarnated. In spite of this carefree attitude, Gengetsu still cares deeply about the fates of the shinobi originating from his own village, lamenting the prospect of having to confront them after his reincarnation. Nevertheless, he tries his best to guide the shinobi in finding the real him once his Demonic Illusion: Steaming Multistoried Building is cast. Gengetsu also had a strong sense of honor, as shown when he immediately reprimanded Mū for thinking of using underhanded tactics after the Fourth Shinobi World War was over.
Gengetsu also seems to have somewhat of a short-temper at times. In the Naruto anime, he has shown great sensitivity about his facial hair, seen from his initial threatening to kill shinobi that made mention of how thin it was, though letting bygones be bygones after an apology concerning this subject. However, when brought up again, the death-threats only began anew. His anger was most apparent when concerning Mu and their enmity, which he seemingly places more value upon than even his own life, aptly demonstrated by the manner of his demise, and his neglect to ascertain the nature of his reincarnation, simply because his killing of Mū was brought into dispute. Part of this enmity seems to be due to Gengetsu's dislike of how selfish and underhanded Mu is, as seen when he lashed out at him for telling Ohnoki that he should be sure to take advantage of the eventual disbanding of the Allied Shinobi Forces. Gengetsu has also taken a great liking to Gaara, referring to him as a "golden egg" amongst the current five Kage, and even refused to help him any further in dealing with his Steaming Danger Tyranny clones, in order to have him prove his worth.