serious
/ˈsɪə.ɹi.əs/
Without humor or expression of happiness; grave in manner or disposition
It was a surprise to see the captain, who had always seemed so serious, laugh so heartily.
Important; weighty; not insignificant
This is a serious problem. We'll need our best experts.
Really intending what is said (or planned, etc); in earnest; not jocular or deceiving
After all these years, we're finally getting serious attention.
(of a relationship) Committed.