Miss Stephanie Crawford is an example of a person who does not know what harm she caused.

Miss Stephanie Crawford is described as the “neighborhood scold.”  She is basically the town gossip.  She either makes up or passes on stories just to seem important, without regard to who she might be hurting.  She shares the details of Boo’s childhood mishap, for example.

According to Miss Stephanie, Boo was sitting in the livingroom cutting some items from The Maycomb Tribune to paste in his scrapbook. .... As Mr. Radley passed by, Boo drove the scissors into his parent's leg, pulled them out, wiped them on his pants, and resumed his activities. (ch 1)

Stephanie also says that Boo was peeping in her window at night.  She loves to pass on Boo' story, even though he is a timid and quiet soul.

Stephanie makes fun of the children too.  She even tells jokes at their expense.

"Whatcha going to be when you grow up, Jean Louise? A lawyer?"
"Nome, I hadn't thought about it..." I answered, grateful that Miss Stephanie was kind enough to change the subject. ...
"Why shoot, I thought you wanted to be a lawyer, you've already commenced going to court." (ch 24)

Miss Stephanie likes to have her fun at the expense of others, and does not seem to know or care of her effect on anyone else.

There are many characters that do harm without realizing it.  Mayella accuses Tom Robinson just to save herself.  Boo Radley’s father keeps him in the house to protect him.  Aunt Alexandra tries to make the Finch children appreciate their heritage, and really just confuses them.  Finally, the Finch children portray Boo Radley’s life before the whole neighborhood without realizing they might hurt his feelings.

Who are three characters unaware of the harm they caused in To Kill a Mockingbird?

Mayella Ewell did not realize that her actions led to the death of Tom Robinson and the destruction of his family.  Mayella, in her ignorance, thought that if she committed any wrongful act it was only that she kissed a black man.  She did not see that her accusations against him and her steadfast refusal to accuse her father of abuse were actions that led directly to Tom's death.  She did not consider Tom a human being on equal footing with herself because she was white and he was black.  Her father, Bob Ewell, was the same way only to a greater degree.  Whereas Mayella was lonely and ignorant, Bob was lazy and mean.  Bob knew he was hurting Tom but he was unaware of how much his racism would hurt himself and his family.  Bob was just too self-centered, dumb, and mean to care.  He also never realized that his actions almost led to the exposure of Arthur "Boo" Radley who was so shy and backward that public exposure would have been very harmful.  Finally, Nathan Radley (and his father) never realized the harm they caused to Arthur Radley.  Because Arthur was never given the psychological help or even the simple understanding that he needed as a boy, he was locked away at home and that led to him being the recluse that he was.