Oni is the code name assigned to a Calamity Class Entity that has existed in many shapes and guises. It seems to have been shaped into being by a being called The End Mother. It is one of Three Calamities born of this being and currently the only one who isn't actively antagonizing humanity... at least for now. It is capable of regenerating from any injury even beheading and can possess human bodies when it needs. It typically only takes a new shape when it's current one has been found. 

Due to Oni's need and love for eating it would usually keep a host for a year to two before having to move on. Often it will select Humans who are already longing for death or vengeance in some way; though it can make exceptions. It tricks these humans into making a deal with it to escape punishment in the afterlife. Once the deal is made it merges with the soul of it's Host and eats the flesh of it's former host before it sneaks away in the night free from pursuit. It can be called a hive mind formed of wicked and hateful beings.

It has a deep longer for a purpose and sense of self that it is incapable of achieving.

Merging Effects and Oni's Current State:

Merging with another being allows the Oni to absorb their memories and Knowledge as it's own. It can then mimic the personality and life of it's new host perfectly, however Oni has trouble telling which host lived through what at time. It also loses memories from past hosts as time goes on or if the previous Host was killed. Oni cannot bring the hosts back to life and apparently his merging to them has altered their future incarnation's (Reincarnation's) fates. His sense of self has been broken down for so long that it cannot recall who the Oni was originally at this point. 

The scars on the Oni's face and torso remain the same from host to host most likely an artifact of the original Oni's appearance. 

Currently Oni has developed a slightly kinder Persona thanks to the original Owain being a child at the time of their deal. Owain was dying of stab wounds caused by his own parents and in that moment Oni appeared before him. The nature of the Boy's wish is unknown but it's effects have caused Oni to take less thrill in hunting then it once did and allowed it to take interest in other things. It has become weaker then before as it is not eating as often, though that has also allowed it to live a normal human life for the first time in the entirety of it's existence.  

Oni is still not a kind being by any stretch and seems to actively force itself to live the life of Owain at time. It has also developed a great fear of being found in it's current form as it has grown to enjoy it's life for the very first time. However it cannot stop eating Human flesh as it's being is incapable of feeding on regular foods effectively.