So with how Senjutsu was explained in DxD and how it effects some people I thought back to how Zestiria and Berseria had it.
To every action their is an equal and opposite reaction. During the Great War the world was basically drowning in malice from all the deaths, the hate, and all the sorrow of those left behind or caught in the middle that one madman decided to try and harness its power but using dragons blood as a catalyst instead created a dragon that embodies the concept of Malice a mindless beast of destruction and ruin. This beast caused the three factions to come together to kill it but the beast was just a heavy concentration of malice so it had no soul yet it simply dissipated leaving a body behind. With the world is full of malice and every so often one will be cursed with slowly taking in the malice of the world till it turns them into a Malevolent Dragon that brings calamity where it treed. So Legends spread that once every thousand years an evil dragon with strength that rivaled that of the Heavenly Dragons.
Seath Ninistr has received this curse when he was born with a birth mark of a smoking dragon on his chest. His name loosely means Beast of Ruin. He was an orphan in Kyoto with wild black hair and red eyes. Growing up Seath was avoided as he was growing up in the orphanage cause of his quick temper and red eyes causing the others to call him a Reaper. Then one day when he is 8 he wonders into a forest to be alone from the abuse the other children do to him and ends up lost till night fall. Lost in the forest he stumbled upon a stray nest in a cave where a few strays were still feeding. As he was backing out he bumped into one where it decided to toy with him before killing him just as he lost his will to live in a world that rejected him his chest lite ablaze in a violet fire spreading across his body with wings of fire and crimson flame eyes burning the area around him. He then releases a roar akin to that of a raging dragon was heard across Kyoto. The hate and malice he exuded was sensed by many senjustu users in the city but the concentration of it caused two specific people to the leader of Kyoto Yasaka and the Monkey King Son Wukong. They went over to the source only to find the trees burning in a violet fire and the corpses of stray devils and their victims. Standing in the middle of the fire was a Seath covered in the violet flames but instead of hand and feet they were slowly turning into massive claws. Seath in his berserk state attacks them on instinct as they dodge they can sense him absorbing the malice of nature around him so stop this Wukong decides to knock him out with a blast of senjutsu to find out what was happening. They didnt expect to see a beaten human child being the source of it all. Upon realizing they stubled upon the next bearer of the curse Wukong advises Yasaka to find someone to train him in senjutsu or at least in Touki as a way to see if it will prevent the next awaking of the beast or prevent it all together if the boy can control or at least suppress and expel the malice he cant handle. Through the years Seath is very well taken care of and trains though while he couldnt use senjutsu without hurting himself with the malice but had mastered the art of Touki. During his 7 years of training Yasaka visited to check on his progress in that time he became something akin to Kunou's big brother. When he turns 16 he transfers to Kuoh Academy as a second year where he kills the occasional strays. Though the Devil in town have been watching him cause Koneko can sense something wrong about his presence.
Fighting style is a mix between martial arts and sweeps from his partially changed his hand(s) into a beast like claw
Likes animals (though they end to avoid him most of the time), peace, silence
Dislike perverts, needless violence, stray devil, bullying
Skills Partial beast transformation, draconic transformation (high chance of losing control but is highly powerful), martial arts expert, Touki mastery
Should he every lose control for too long or no one filters out the malice he absorbs he will fully transform into an Evil Dragon that will constantly get stronger until he is either killed or someone with senjutsu knowledge is able to transfer the malice to someone else (being boosted also boosts the malice and dividing will transfer it to the wielder of Divine Divide)
Ill be honest not the best but its just a basic idea feel free to change the backstory