Sentence:  His addle-brained knight-errant , self appointed to the ridiculous position in an age when armor had already been relegated to museums and the chivalrous code of knight-errantry had become a joke , <WSD>is</WSD> , as Cervantes no doubt intended , a gaunt but gracious symbol of good , moving soberly and sincerely in a world of cynics , hypocrites and rogues . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)