You just do it like the thousand shows that already did it.
Obviously budget is limited, but that is really not something new?
Colour, details, shadows, lack of shadows.
I mean, come on, I literally posted the LN version of the same character who looks completely different and has a completely different aura to him.
Anime has extra benefits from getting to choose his voice and the way he moves etc.
You could add twitching head movements and "clacking" noises when he moves and all that stuff.
Not to mention that you can have "chibi" version and whatnot if you want them to picture the daily-life stuff, to emphasise the contrast between what he is and what he does or what is alignment is.