This is an ability of Tooru's as a Slavemancer, the first ability that he gains along with his Title as Slavemancer.

This ability level indicates how many people he can control, and as this rises, so does his slave capacity.

Before his enslavement magic skill reached LV 6, all his slaves "had hollowed eyes which, in layman terms, was the look of a woman who was raped until she had lifeless eyes" and were "acting merely like robotic dolls without emotion", but once he reached said threshold, meaning LV 6, he obtained a new ability.

This was, the ability to give his slaves “their former personalities” back — naturally, the only difference from their original personalities is the fact that they must recognize Tooru as their lord/master and serve him with absolute respect and obedience. Now, instead of having lifeless eyes, they all have a faint, heart shaped pattern that seemed to appear within their mesmerized and intoxicated eyes.

Heartshaped eyes

Personal Observation

It feels like those without a degree of magical resistance will be more open to their sexual drives.

Those with, would depend on their "level of innocence", so to speak. Flamia is the only one with high magical resistance that willingly embraces her sexual desires. Palmyra is the second, mostly due to her dependence towards magical sustenance.

The only other individual, is Hero Gal Riruna, but this is perhaps because of her own liberal "Gal" way of looking at the world.

The only two who remain adamant regarding their virtue or rather, their lack of indulgence of their own sexual nature and giving in to Tooru's whims, specially in public, are Kirika and Celesta.