I'm working on a rakshasa villain with a mantis head named Yeren, he is the primary villain of the campaign with the main party being in steep dept to Yeren. I'm thinking of ideas for him a few ideas I'm not sure on yet are Yeren having a tiefling child who masquerades as a thri-kreen who lost his second pair of arms, Yeren having a romanticish interest in a neutral good woman that he keeps around officially to scare off other rakshasa due to her abilities, and him having a section of his city where he hangs the partially eaten bodies of people that could not  ever pay of their debts. The ideas I'm sure of Yeren having a dislike of demons and  devils and instead uses yugaloths, the city that Yeren controls is in-debt to him, and instead of him being a sorcerer he is a monk.  Any ideas or suggestions?

I just realized I may have posted this in the wrong section. 

Quote from KurtalmakTheKoboldKaiser >>

I just realized I may have posted this in the wrong section. 

 Yep. No problem, though.

Thank you for relocating this thread.

I believe I have his personality down with him presenting himself as a calculating and psychopathic but if you are one of the 3 people he genuinely trusts you will find that unlike other Rakshasa he generally thinks low of himself with a less explosive anger than most, but he is still a psychopath.

I think you’re overthinking the bad guy here. Are you sure this isn’t getting into DMPC land? 

The internal motivations are fine to acknowledge in a general way, this Rakshasa is transactional and possessive, but taking it down to various scenarios and the internal thoughts he has vis a vis his assistants and employees isn’t something that players are going to be encountering, much if at all. 

Your time is almost always better spent in preparing for the adventure, rather than imagining how the villain thinks, because his thoughts aren’t going to help much, his actions per the players are. 

I agree with Mr. 1975 here :)

With any NPC, I find all that's really needed to know is what their motivation in the situation, what their next goal is, what archetype ( or alternatively a non-standard "3 word description" is - like Nature Oriented Dwarf ) they are, and maybe a distinctive quirk or two.

With more complex or important NPCs, I could see expanding this out to: what are their motivations and goals in this conflict; what resources do they have at hand; what are their abilities; what do they know; what do they believe; what is their personality; what are their tactics ( what do they like to do, what will they only do in an emergency, what will they never do) - and from all this: what is their next planned move. All this is taken at a pragmatic level only. For their personality, I only need to know that the evil Wizard is condescending, arrogant, and cruel - not why.

This doesn't mean that there's no chance you will ever need to present, or know, their internal motivations or personal details - but either a) it will never come up in the game, b) you can invent these details on the fly, and you can jot them down as in-game notes. Spending effort on up front design is perhaps not the best use of your energy.

If these sorts of details just spring into mind as you're creating NPCs - that's great, but I wouldn't even write those down, and just keep them in mind. That way, if there comes a point in the story where they become relevant, you can use those ideas, or something else entirely that fits the story better. But if you've written them down, you may feel bound to use them, even if something else would make for a better narrative.