Akira Fudo is a teenage boy who is a bit of a coward, he wishes not to involve himself in fights and would rather run away. He also is known to cry a lot.  One day Akira meets with his friend Ryo Asuka. Who informs him of the existence of demons. After seeing the proof for himself, he agrees to help fight them. Akira is then possessed by the demon Amon. After that, he becomes Devilman, and his whole personality changes. Akira now loves to fight, and becomes a snarky tough guy. But the true Akira is still somewhere in him, he still cares for those he loves. In his Devilman form, he has amazing strength, can fly, can shoot lasers/fire, and can also use his eyebrows as tentacles. 

Voice wise:

Akira pre possession, is rather whiny, a bit like Morty if he was pitched down, he has a relatively medium voice, not too high, but not too low. 

Akira post possession, is the same voice but his attitude is different, he talks really raspally and like a punk, but keeps his medium voice, during emotional moments, he slips back into his pre possession voice. 

Devilman, sounds like a complete beast. He has a lower medium voice, this time sounding deeper. He talks very animal like, he growls a lot, letting his animal nature take him. His yells are powerful and very intimidating.