I start with something that's been on my mind, either to satirise it for character beliefs or as inspiration and vibes
(for Lernaean, it was wealth inequality, the billionaire social calendar, Gossip Girl & Succession and interest in making a character to reflect in a Bizarro-esqe juxtaposition compared to their related family characters) and (for Jiangshi/Driver, it was sort of the lonely warrior/samurai or old man Logan esqe trope, I had just watched the John Wick franchise and Seven Samurais for the first time and Old Boy a bit later on and had seen a lot of noir movies with older protags which made me want him to be considerably older than the contemporary player as well as to just switch it up.
I think characters are always better if you can in your mind mimmick their distinct voice. Does your character think before they talk? How's their accent? How good would their vocab be? Do they have little ults they pull, Do they know Japanese at all? Like you after 3 or 4 interactions with my character should be able to write dialogue for me and how much so? And the more so the better I end up feeling I did
With Lernaean, I sorta went with a Sheldon Cooper/Brainiac approach for a while before giving him "bug eyes" to make the nerd angle more glaringly obvious. Later on I toned that down and kept up with the fish out of water vibes in other ways such as odd hat wear and inability to socialise
With Jiangshi, he has two distinct personas one for criminal life as Driver and one for himself. I considered this a sort of dual personality Jackyll & Hyde/Batman sorta thing. Driver can only speak in monologues or not at all, is slightly politically backwards as he was formerly a cop, and then as a citizen he was a Evil Miyagi father,, unavailable emotionally to his children in the forms of missing obvious cues or being unable to adequately explain emotions and then as a smoker but with a sort of Homer Simpson "it all works out in the end of the episode because they love each other" appeal to his relationship to them. So it's either played for casual laugh or its a more emotional arc for the characters and that changed on a whim