Which term is used to describe someone who is affectively modest or shy in a playful way?

The term that best describes someone who is affectively modest or shy in a playful way is "demure." This word conveys a sense of calmness and reserve, often used to describe someone who behaves in a modest or shy manner, especially in social interactions. It carries a gentle connotation, suggesting an innocent or playful bashfulness that can be endearing.

In contrast, the other terms do not fit this context. "Pompous" refers to being overly self-important, while "cavalier" implies a dismissive attitude or lack of concern. "Decimate" means to destroy a large portion of something and does not relate to personality traits or modesty at all. Hence, "demure" is the most suitable choice for describing someone who is modest or shy in a playful manner.