It occurred to me that I actually have a copy of the Encyclopdia Onmyoujica Okaniensis, so I rousted it out from downstairs and squinted at the kanji to see what it had to say about True Kudzu aka Murasaki Sue. And stopped reading after the first three words, because the first three words were 'nazo no shoujo'. (Well no, they were 'sees demons and gods', because Japanese works that way, but you get the idea.)
Nazo is 'riddle, mystery', no is the connector, shoujo you already know. And just as, in Japanese and Mommy-speak, 'maybe' means 'no', so in manga-speak 'nazo no' means 'I'm not telling you anything.' So much for that.
Of course I did read on and learned that a) she's an original character, which is only too obvious, and b) 'though peripheral to the action of the first seven or eight volumes, in the later ones she becomes the woman who is essential to Seimei's life' oh spare me. Harriet Sayers Syndrome strikes again.
As for 'I do not love thee, Yasunori', the Encyclopedia says coyly 'seems that there was 'something' between him and the young Seimei.' Erm yes, that too, going by those cuts of furious-looking young Seimei saying 'Don't touch me'. Okano is very good at suggesting sexual horrors exactly where one doesn't want them ie between the General and whoever her father is.