Kokoro's development of her own emotions was an idea suggested to her by Mamizou, and something she's doing very consciously and deliberately, largely one emotion at a time. At the time that she lost the Mask of Hope, several religious groups capitalized on the hopeless malaise over the village for their own ends. Her original plan for establishing her own emotions was to simply interact with people (by fighting them) and seeing if her interactions with them can trigger any emotions in her, but as she submerges herself in the situation and learns about how people have been reacting to the incident, she had her first real personal emotional reaction - anger at the three religious leaders for exploiting her situation. In the aftermath of the ensuing battle, she accepted that she would be able to develop her own emotions, and the ensuing feeling of hope is what allowed her to get by despite never finding her Mask of Hope.
Since then, Kokoro has been emotionally experimenting, learning about specific emotions in the hopes of becoming able to feel them herself, with occasional successes. During the second incident she took part in, in which she was co-opting the identity of the urban legend Kuchisake-onna to shock and surprise people, she was eventually having a bit of fun with it - in one or two of her expressions during the event, she can be seen to be smiling faintly (such as in the above icon). At the end of that incident, her urban legend persona led her to an unexpected battle with someone from another world, who used techniques and powers she had never even imagined; through this last experience, while she was trying to surprise others, she ended up experiencing the emotion of surprise herself.
The emotions embodied in her self aren't the same in nature as the emotions embodied in her masks. They're more nuanced, more fluid, and harder to control. With the masks, Kokoro's experiencing of emotions was almost conscious - experience something, decide that it's something she should be angry about, draw upon the Mask of Anger. Without the masks, she experiences them more like a human, just becoming angry because she was wronged, with no steps in between.
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Since then, Kokoro has been emotionally experimenting, learning about specific emotions in the hopes of becoming able to feel them herself, with occasional successes. During the second incident she took part in, in which she was co-opting the identity of the urban legend Kuchisake-onna to shock and surprise people, she was eventually having a bit of fun with it - in one or two of her expressions during the event, she can be seen to be smiling faintly (such as in the above icon). At the end of that incident, her urban legend persona led her to an unexpected battle with someone from another world, who used techniques and powers she had never even imagined; through this last experience, while she was trying to surprise others, she ended up experiencing the emotion of surprise herself.
The emotions embodied in her self aren't the same in nature as the emotions embodied in her masks. They're more nuanced, more fluid, and harder to control. With the masks, Kokoro's experiencing of emotions was almost conscious - experience something, decide that it's something she should be angry about, draw upon the Mask of Anger. Without the masks, she experiences them more like a human, just becoming angry because she was wronged, with no steps in between.