Aunt Clara is Lennie Small's aunt who adopted him as a baby and cared for him until she died.
She placed George Milton with Lennie's care and told him to take care of her nephew. She doesn't actually take place in the story other than in memory and in the last chapter where Lennie hallucinates. She did not understand Lennie's disability but gave him mice and soft things to pet. She is described [in Lennie's hallucination] as "a little fat old woman. She wore thick bull's-eye glasses and she wore a huge gingham apron with pockets, and she was starched and clean." (Steinbeck, Of Mice and Men 96).