The placidity in his nature makes him the most popular mediator in the team.

placidity 是名词，来自形容词 placid（平静的、温和的）。词根 plac- 来自拉丁语，意思是"平静、安宁"，加上形容词后缀 -id 构成 placid，再通过名词后缀 -ity 变成名词 placidity，表示"平静、温和"的特质。