Ok so this is when Izuku has a fire like aura cat that comes out of his chest. A lion named Netsu. Izuku has cat ears and a tail. He can also manipulate fire from his body like endeavour. But it can burn his insides, cause him to overheat, gives him burns, lost of energy and whenever he feels pain Netsu feels said pain. Netsu is a lot like dark shadow but basically he replaces Izuku heart. Izuku also has a second digestive system and it helps him store energy. Whenever Netsu is near fire or Izuku is feeling overwhelmed with emotions. He turns into a monster like Dark Shadow. Netsu is Izuku prime protector and Izuku emotions inflicted on him. If Izuku feeling pain in his arm he feels pain in the same area. If Izuku loses a limb. He replaces said limb. Netsu can't get near water as it will turn him to ash. Izuku also can get dehydrated much easier. Any Scar Izuku has Netsu has as well. 

This is a mixture of Hellflame and Dark shadow basically but more advanced 

The heart is what on Izuku chest. This is where Netsu comes out of.

The lion with the light green and orange eyes is Netsu in his normal form, this is what he looks like when Izuku emotions are good and there no flame around them.

Netsu with red eyes is the Feral Netsu. This is when There are flames and Izuku emotions are overwhelming. He is completely hard to control and grow as big as a building.

The eyes are what Izuku eyes will be like when Netsu go feral. He will feel everything Netsu is feeling. He will hear everything Netsu hears. Anything that happens to Netsu happens to him now...the roles are reversed

He wears a collar to hold Netsu back