The idea was that Nero could upgrade his DT, and he could upgrade it to level 3 for each Devil Arm. Like say for one of the weapons I made up, which was a lava katana named Phantomtail. Look at this chart.
Level 1: Nero has a humanoid ghost of Phantom behind him, and he has a burning aura of fire surrounding him.
Level 2: Nero's skin is slowly becoming like stone, he has grown Phantom's mandibles, his limbs are slightly longer, he runs slightly faster and six small knubs are poking out of his sides. His burning aura has also intensified.
Level 3: Most of Nero's clothes except for his pants are burned off. Nero's head is very bestial and insectoid, with mandibles and extra eyes. The six spikes that were coming out of his sides before are now fully functional extra legs which he can use to climb specific walls. His skin looks like stone now. The aura around him will slowly damage enemies who come in contact with it.