Monks is greedy and manipulative.
Monks is Oliver’s half-brother. His intention is to ensure that Oliver cannot inherit from his family, so he tries to make sure that he never finds out who his real parents are. In order to do this, he is in league with Fagin, the fence and all-around criminal, to turn Oliver into a criminal.
Monks is not a very good person. Instead of telling everyone that he knows where Oliver is, he keeps his identity a secret and tries to ensure him a life of crime. One could assume that he might have been handsomely rewarded for letting someone know where Oliver was, but instead, Monks chooses this nefarious path. This shows him to be greedy.
'I tell you again, it was badly planned. Why not have kept him here among the rest, and made a sneaking, snivelling pickpocket of him at once?… If you had had patience for a twelvemonth, at most, couldn't you have got him convicted, and sent safely out of the kingdom; perhaps for life?' (Ch. 26)
The fact that Monks intended not only to keep Oliver’s identity a secret but also make him a criminal shows that he is manipulative. As this shows, he was hoping to get Oliver out of the country and out of his way. When Fagin says he was not like the other boys, Monks replies, "Curse him, no!' muttered the man, 'or he would have been a thief, long ago" (Ch. 26).
Monks goes to the workhouse and talks to Mr. Bumble, trying to find evidence that can lead back to Oliver’s birth. There, he finds the locket.
And, as if glad to be relieved of it, she hastily threw upon the table a small kid bag scarcely large enough for a French watch, which Monks pouncing upon, tore open with trembling hands. It contained a little gold locket: in which were two locks of hair, and a plain gold wedding-ring.
'It has the word "Agnes" engraved on the inside,' said the woman. (Ch. 38)
Although Monks now has in his possession the only proof of Oliver’s identity, fortunately, things do not work out well for him. Brownlow is able to make him confess the whole sordid affair about their father and Oliver’s mother, and Monks’s plan. Oliver can now return to the noble birth he is meant to have.
Oliver remains good throughout the story. Monks (whose real name is Edward Leeford) is an example of the anti-Oliver, a sort of foil. He is rotten through and through, full of greed and envy. Even though Oliver has had a difficult life, Monks is the one who despises him because he is good. He curses Oliver for failing to become a criminal, while he, Monks, has descended from his noble birth to consort with them.