I've been struggling to figure out what Yowapi should be like visually and fundamentally.

This is what he looks like.

His backstory is quite complicated.
"Eons ago, he was born to a royal family that held great power. On the land that was there, the people were treated with enough respect, having very few troubles with the order. When he was being educated about ruling, he was rather unamused with some things being lackluster to his liking. After successfully finishing his portion of schooling, he went through a process of initiation, becoming the rightful prince of society. Little the family knew, Yowapi was hiding a darker side to his persona. After planning a proper execution, he hired a spy to slip poison into his fathers drink. By the time his father passed away, he was given the title of ruler. Weeks later, he had his mother murdered before he put his new world order into play. It began with laws that caused the people to panic and rebel, thus making his reign of terror just the beginning of greater threats."

Cool, I like the backstory and how simple yet evil he looks on the portrait, though I think it could be improved.

Which anime art style should I use?

I'm thinking of sticking to the DBZ style since I feel it fits.

Judging by his backstory, he seems to be more like an evil person with power over society than a super-warrior, so i wouldnt use the dbz style but stick with the one used in the portrait.

Here is an updated version of the portrait.

I'm trying to come up with some transformations while making him.