Tsukasa really is here for him. There is nothing sinister about her presence. I am actually, one hundred percent serious. She has mysteries, but they just help to inform why she is here, at this moment, with him, right now. It was pure chance and fate that led to him meeting her that night two years ago, but everything after was her waiting for him to be legally capable of marrying her, finding him (to be honest, it's probably not hard to look someone up with public records), and officiating their marriage, just as they agreed on the night they first met.
This manga is ultimate fluff and sweetness. Appreciate it.
This message brought to you by one who enjoyed the entire anime and is re-acquainting himself with this lovely tale from the very beginning again in comic format.