In Chapter 8, while Miss Maudie's house is on fire, Boo Radley puts a blanket on Scout's shoulders because it is so cold outside. Scout was so mesmerized by the fire that she didn't even notice it was him. Atticus tells her: 

“Boo Radley. You were so busy looking at the fire you didn’t know it when he put the blanket around you.” 

Recall that Miss Maudie, in Chapter 10, says that a mockingbird causes no harm and only makes music for others to enjoy. Therefore, a character who is comparable to a mockingbird is one who doesn't harm anyone and only tries to help others. Boo does this with the blanket. 

Miss Maudie is also a mockingbird character. She is always helpful and understanding with the children. Even at this moment in Chapter 8, while her house is burning, she is thinking of others. Scout marvels at her generosity. 

Miss Maudie puzzled me. With most of her possessions gone and her beloved yard a shambles, she still took a lively and cordial interest in Jem’s and my affairs. 

The mockingbird character tends to be associated with being helpful, harmless, and also powerless. This is why Boo and Tom Robinson are the main mockingbirds in the novel. However, if we just focus on being helpful and harmless, Atticus fits the description. In Chapter 11, he describes his perspective on people in general, beyond racial or social labels. He says, "I do my best to love everybody . . ."