He "s a very serious person."

他是个很严肃认真的人。

youdao

The President's seriousness has been demonstrated, and he is going to continue to be very serious about it.

WHITEHOUSE: Press Briefing

He handled his son's drug problem very successfully, and defused matters before they inflicted serious political damage.

BBC: Is Blair a butcher?

"It's a very serious situation still with the U.N. in Algiers, " he said.

CNN: Rescuers search for bomb survivors