To Conway, what is the most mysterious thing about Manchu? (Chapter Six)

From the text, we can infer that Conway finds Manchu to be a complete mystery, although, it does seem the inability to guess her age is the most mysterious thing about her.

It was impossible to know what was in her mind; it was difficult even to guess her age. He would have doubted her being over thirty or under thirteen; and yet, in a curious way, such manifest unlikelihoods could neither of them be ruled out as wholly impossible.