Isabella is a novice nun whose first words in the play are to wish for “a more strict restraint” upon the sisters of her order. This zeal in her vocation is continually evident. It is obviously a source of pain to her that the fault for which Claudio is to be executed is the type of sin she herself most dislikes. She makes this point to Angelo as soon as the two meet, and her prejudice makes her plea for her brother’s life particularly cold and ineffective:
There is a vice that most I do abhor,
And most desire should meet the blow of justice;
For which I would not plead, but that I must...
Isabella is willing to give up her mission remarkably easily, and it is only Lucio, a thoroughly sinful man by her estimation, who persuades her to continue. Finally, Angelo suggests to her that there might be “a charity in sin” to save Claudio’s life. Isabella immediately replies,
Please you to do't,
I'll take it as a peril to my soul,
It is no sin at all, but charity.
When Isabella thinks Angelo is talking about a sin which he might commit to save her brother, she is enthusiastic about accepting his sacrifice, even saying that she will pray to take the sin upon herself so he will not have to answer for it (though she must know that, as a matter of theology, Christ alone can do this). However, when she realizes that Angelo is not offering to commit a sin on Claudio’s behalf but inviting her to commit one, she recoils in horror. She is perfectly willing to accept a sacrifice from him and to proclaim that it would not really be a sin at all. When she is asked to sacrifice herself for her brother, however, she rapidly decides that such a sacrifice would be a terrible sin and that he must sacrifice his life for the preservation of her virtue. This is the most glaring contradiction in Isabella’s character.