Being invincible is impossible, so please change this.
Genstu was born in Mid 10,000 BNG and when he was just a few months old, Genstu was subdued to Demon Energy, being possessed by it. But it let him live a normal life... like a real baby does.
Birth of Little Brother Kybestu In 1BNG, his brother Kybestu was born. Genstu was not pleased, at all. He intended to kill his new brother, than decided not to. Instead, he waited for his 1st birthday. When that came around, he implanted his brother with his demon's kinder side, knowing full well that he would be over ran by the evil within his second soul. But that didn't stop him, he continued to implant his brother with it.
Battle on Death Cannon
On Death Cannon, Genstu waits for his little brother; Kybestu. As Kybestu was running to Death Cannon, he was ambushed by ki balls. After dodging contless ki balls, he arrives to the cliff and Genstu possess a challenge. Kybestu unwittingly excepts and they battle. Throughout the battle, the demons within Genstu and Kybestu, start to build anger for each other. Genstu swated Kybestu away and transformed into a demon. Knowing full well that Kybestu won;t be able to defeat him, he releases the demon within. Genstu uses his full power and knocks Kybestu off his feet and into a mountain. This sparked the anger within Kybestu to go even further. He transformed into more of demon. Kybestu unleashes his most powerfullest punch at Genstu. He flew into the mountain and gets up, he fires all of his energy at Kybestu, but low and behold, Kybestu was not affectited. Genstu charged for Kybestu and Kybestu charged for Genstu, the two clash and are repelled by their oppsoing powers. Kybestu then states: "We are at an equal power level, no sence in fighting, if we can't kill one another." This didn't stop Genstu. He reached into his inner demon counterpart and pulled his full demon out: which can not be stopped unless he kills his target; hense Kybestu. Kybestu unleashes his ultimate attack: The Demon Galick Kamahamaha and it destroyed most of Death Cannon(not going vertical, going horazontal.) Creating more cannon to fight in. Genstu didn't let an enormus blast stop him from achieving his goal. He took out one of his six swords and acitvated its True side: An enormous blade that is more heavier than Bankostu's Banryu from InuYasha. Genstu raised it and slammed it on the water, sending a huge wave of air or Demon Slash Wave. It misses Kybestu by a hair and Kybestu used his own Demon attack: Demon Counter Wave. It cuts Genstu's arm(cut the elbow in half) in half. This got his even angeryer, he regrows his arm and fires his Demon Spread Fire, which fires multiple ki balls at the same time at an enemy. Kybestu dodges them all and finally charges up a huge Demonic Galick Gun in one hand and punches him on the stomach and blasts him into the mountain. Forcing him out of his demon rage.
Genstu has retained Immortality. Genstu has been known to because of the demon within. But how and/or who gave him immortaility is unknown.