I think this is why Kirara likes Kohaku so much. Half-orc lady treats her like an idol and exorcist lady treats her like she's coddled family but Kohaku just treats her like a friend and is even thankful for her being one.
Medium is her size, but its also her attitude. She represents a kind of middleground between the other two for kirara, which i imagine is why she looks upon her so favourably.