单纯 describes a person's character or nature as simple, pure, and unsophisticated — often with a positive sense of innocence. Unlike 简单, which is used for tasks or things that are 'not complicated', 单纯 focuses on a lack of worldly experience or cunning. In its adverb use (e.g. 单纯是), it means 'merely, purely' and is less intense than 纯粹.