Since a thesis statement is an answer to a particular question that can be explained and supported with evidence from the narrative of Harper Lee's novel, you may wish to use Atticus as the character who best exemplifies the above quotation. For, in Chapter Five, as Miss Maudie "ravels a thread" as she ponders what goes on behind the closed doors of the Radley's house, she explains to Scout and Jem,

"...Atticus Finch is the same in his house as he is on the public streets."

In other words, there is nothing pretentious or hypocritical about Atticus Finch.  That he is honest and forthright always and possesses great integrity is evidenced in several scenes:

Atticus is a gentleman and treats everyone fairly, giving them respect. 

• He scolds the children for subjecting Boo Radley to their ridiculing games. In Chapter 5, he scolds the children,
  ..."stop tormenting that man."  Atticus insists that what Mr. Radley does is his own business.
• He chides the children for their snow caricatures of Mr. Avery and other neighbors in Chapter 8.
• He tips his hat and is respectful to Mrs. Dubose despite her insults.  In Chapter 11 he explains to Jem that she is old and ill, and speaks politely always to her.  Scout notes that at such times, she thinks her father "the bravest man who ever lived."
• He defends Calpurnia against his sister Alexandra when she wants her brother to dismiss the maid.
• He treats the night vigilante committee with valor, facing them in the light of the jail. 
• He speaks in the courtroom in a respectful tone to the hostile witnesses, Mayella and Bob Ewell and Mayella, even addressing her as "ma'am." 
• He defends Tom Robinson against the mores of his society.  Explaining that he must defend Tom because in a court of law everyone ie equal, Atticus tells Scout,

"This case, Tom Robinson's case, is something that goes to the essence of a man's conscience."

Perhaps, as a summation all of Atticus's fine attributes, Miss Maudie instructs the children,

 "If your father's anything, he's civilized in his heart..."

Atticus is, indeed, "civilized in his heart."   He believes it is a sin to bother those that bother no one; he thinks it wrong to misjudge people for taboos or for ulterior reasons.