se·ri·ous
ˈsirēəs/
adjective

1.
(of a person) solemn or thoughtful in character or manner.
"her face grew serious"

synonyms: solemn, earnest, grave, somber, sober, unsmiling, poker-faced, stern, grim, dour, humorless, stony-faced; More

2.
acting or speaking sincerely and in earnest, rather than in a joking or halfhearted manner.

"suddenly he wasn't teasing any more—he was deadly serious"
synonyms: in earnest, earnest, sincere, wholehearted, genuine; More