I'm trying to write a villain but don't know how to do it properly

So my campaign is going to be set in a world a friend of mine first set up that I'm now picking up the mantle of. The villain I'm trying to write will appear mid to very late game. Basically the villain of the previous dm was a lich who was disguised as the previous king's aid and then framed the prince for his murder and took over control of the kingdom. The lich's goal was to combine all of the planes together and then kill everything. He was defeated and a new king was put in place. This is where my problem begins, I want the villain or maybe not exactly a villain to be the king. Basically anytime anyone asks how long the current king has been in power they automatically respond with he's always been here. He'll be a rather unknown person, even by the royal guard. He has not taken a queen and his royal aid does most of the communicating for him. I have yet to understand what motivates him? Power? but he hasn't done anything tyrannical and the kingdom is actually flourishing now especially since trade and communication with the other planes has become significantly easier (I can go more in depth later if asked for). I'm not even sure what race to make him. If anyone has written a similar villain could you please help me out here I'm struggling out here.