Originally Posted by
shinta|hikari
When a devil takes over a human corpse, it becomes a fiend. If it doesn't become more powerful, why do that? Why not stick with its original devil form? At the very least, it seems to remove the need to consume blood, since Power doesn't seem to need to drink blood to maintain herself. That's where I got the fiends are stronger than devils, at the very least if its the same devil becoming a fiend. Naturally, there can be variances where a fiend is weaker than an originally high level devil.
As for Pochita giving his heart to Denji, that's because he cared for Denji and wanted to see his dream, even if Pochita himself ceased to exist (in his current form) to do so. If Pochita just wanted to survive, he could've possessed Denji and become a fiend, like Denji himself suggested. Denji is NOT a fiend. His case is special, as Makima mentioned.