Man, when i remember that the first time i read this series i immediately dismissed this because i thought that Seth was cliche shounen MC... i feel so happy in the end i resolve to read this again.
This personality of Melie is a little strange to the continuity ( to me at least ) if this is just a third personality all would be easily explained but for some reason, i don't think this is the case, i think that violent Melie is yandere Melie... i think that the shock of failing and getting her sister killed altered her double personality, this one clearly loves and wants to stay close but does it in the yandere fashion the one we know tries always to get people to go away using her violent outburst, if this theory is right Melie infection can change to accept people as she matures as a person... or her infection is a little more complicated and things can get even more screwed up.