Kiju can sense the abyssal blood within him even with his disguise intact.  She knew exactly what he was when she first spotted them and that was why she was on edge from the very beginning.  I did warn you at the character creation phase that choosing a tiefling for this campaign would be very difficult even with his magical disguise.  It may fool the common NPC, but there is still a portion of those you will come across that can easily see through it.