expression, godan verb (-ru)
look stern; put on a stern face
Describes a person's expression becoming stern or severe, often in response to something serious or displeasing. The phrase is a fixed expression combining きっと (sternly) and なる (to become).
先生はいたずらを見て、きっとなった。
The teacher looked stern when she saw the mischief.
彼は急に
He suddenly put on a stern face and fell silent.