引用自 Nerrevar：

Does anyone know how it what the exact requirements are for the mind control option to show up on rulers?

It doesn't show up or is greyed out? It should show up unless someone already used the mind control on the character.

Yeah the mind control doesn't show up sometimes, so I was thinking there were some requirements for it

Bit late to the party but they need to have a "weakness" mentally speaking. So a vice or a character flaw that you can exploit. Virtuous or otherwise "strong-willed" characters can't be mind-controlled.

So a brave, temperate, just ruler can't be mind-controlled. But if he one day becomes lustful, wroth, greedy or craven you can try to mind control him.