Got a tricky question for you!

While playing with my bara/knights against a house bolton deck, my Ser Eldon Estermont card changed control. In the last round my opponent attacked with Ser Eldon Estermont with 3 powers on, due to his ability(While Ser Eldon Estermont is attacking, characters with fewer power than him cannot be declared as defenders) my knights couldnt defend him. My active plot though was Tourney for the Hand (Each of your Knight characters gets +1 STR and is immune to opponent's character abilities), does the plot's text refer to passive abilities too?

Card abilities are defined as:

" ( 3.7) Card Abilities
"Card abilities" (i.e. "Character ability,"
"Location ability," or "Attachment ability")
refers to anything in a card's text box, except
for traits, keywords, and flavor text. "Card
abilities" also refers to any abilities (again,
keywords and traits are excluded) gained by
card effects"

So yes, passive character abilities are covered under "immune to character abilities," too.