。
She is a very serious person.
Literal
She [topic-は] very serious person is.
まじめ ('serious, earnest, conscientious') is one of the most common compliments in Japanese — applied to students, employees, anyone reliably putting in genuine effort. The connotations are entirely positive in a way English 'serious' often isn't; calling someone まじめ celebrates their integrity rather than implying dullness. Often written 真面目 in kanji though hiragana is also fine.