I'd like to express the full sentence: She looks quite thin, but in fact she is a man of character. Can we apply the expression a man of character when a woman has a strong character?
Unless specifically wanting to address gender variability, the contradiction of using she/man or he/woman should be avoided.
She is a woman of character.
However, this can be simply solved by using gender-neutral nouns.
She is a person of character.
Both are acceptable and readily understood.
More to the point though, it seems that you want to give this particular sentiment:
She is a person of substance.
She is a deep person.
She is a person of conviction and strength.