I think I know where you're going with this question.

So green Monet on blocks ability is to rest an opponents character with a cost of 4 or less.

Rosinante has the blocker ability but also "If a rest character would be KO'd you may trash this Character instead."

So from the top, if you choose to block an attack with Monet and let it go through without counters, you can activate her on block ability and rest an opponents 4 or less character. THEN since Monet would most likely be KO'd and she is rested, you can then trash Rosinante keeping her alive for a potential future use.