Both Doodle and Boo Radley are considered substandard by family members who are embarrassed by their mental and physical weaknesses. Boo stepped outside of his comfort zone to take care of Scout and Jem while Doodle stepped outside of his physical comfort zone to please his brother. Doodle is the scarlet ibis, and Boo is the mockingbird.
Both Boo and Doodle live on the outside of what society has called "normal". Both characters are strikingly different from everyone around them.
For both characters, it is their differences that people fixate on--for example, Doodle's brother can only focus on how DIFFERENT they are and the things that Doodle can NOT do. Same with Boo--the kids focus on the way that his life is DIFFERENT from their own.
These characters are different though in a few respects, too. Doodle dies as a result of his brother's inability to accept him, but at the end of TKAM, Scout does accept Boo just as he is. Doodle's brother does not mature, whereas Scout does.
Both works can operate under the theme that "people may not always be who they first appear to be".
The primary distinction between Boo and Doodle is that Doodle is clearly identified as not "all there," a physically and mentally disabled boy. Boo may or may not have an affliction, but the author only hints at whether he is truly emotionally disturbed. Boo's actions in protecting Scout and Jem indicate otherwise, despite the rumors and innuendo floating around town.
A strong similarity between the two characters is their love and care for those in need. Doodle tenderly buries the dead scarlet ibis; Boo leaves gifts for the children to find and carries Scout back to her home when she is trying to escape from Mr. Ewell.
Both characters symbolize the outsider, the individual who exists at the fringes of "normal." Both are sacrificed to satisfy the desire of others for them to behave normally, Doodle by Brother and Boo by Mr. Radley.
A comparison between these two characters is very interesting to consider. They do have some striking similarities, in addition to their both being tagged with unusual nicknames. Both Boo and Doodle are gentle, loving souls who mean no harm to anyone, but both are sometimes tormented by others, certainly without cause. Boo and Doodle both are well acquainted with living in fear, and each of them suffers because of circumstances beyond his control. Also, Boo and Doodle are rejected and isolated in several ways. There is loneliness in them. Finally, Boo and Doodle each reach a time when they push themselves past their fear and weakness and act with great courage and determination, motivated by love and fear.