When Eiji dumped Spoiled-chan and professed love and faith to Rin, the resulting shock served as in 'impetus' for the 'personalities' to emerge and gain self-consciousness. The vessel we know as 'Rin' activated a defense mechanism unknown to anyone in order to protect the 'existence' of cheeky-chan, spoiled-chan, tsundere-chan and cool-chan (among others??)