I'm mostly confused by the words "May Not" on Nessecary Evil. It prevents choice, which confuses me now especially since so many cards say "choose"
Necessary Evil
Open: If you control a Personality, target a Personality and pay Gold equal to the number of Personalities you control, or 4, whichever is higher: The targeted Personality may not assign to battlefields.
from Rules.L5R.com
"May not" is a wording that prevents a choice, rather than a cost or effect
However, if something is required to be chosen, a player is not allowed to choose an option that is currently prevented by "may not” if there is another legal option to choose.
Ambush
Limited: Choose your performing unbowed Personality and target another player's Personality: Create a battlefield (not at any province). Assign your Personality to attack there. Assign the target to defend there, even if he is bowed. Other Personalities will not move there. Fight a battle there (after this action's resolution). After the battle ends, lose 5 Honor.
So if somebody uses Necessary Evil on my guy, then I use ambush. Am I (A:) allowed to choose him because the costs don't say 'performing unbowed personality who may assign to battles', or (B:) must I choose another legal option because of the words "May not" on Necessary Evil prevents him from being a choice to receive the effects. (provided there is another legal choice)
(its the same if I use Necessary Evil on myself and then use ambush preventing myself from assigning while still forcing them to defend, right? One says 'choose' and the other says 'target' which in theory could suggest the 1st one is more of a 'choice' to assign and the 2nd is more of a 'and apply these effects to that guy you don't control'.)