Kokoro is a child who lives in the same neighborhood as Jahy.

Kokoro is a nice girl and seeing Jahy looking for something, she decides to help her out. She believes her story of the magical gems and finds it cool that Jahy can transform. Even though she wasn't able to help her find a gem, she found a four leaf clover, which she gifted to Jahy, wanting her to be happy and have a good luck.

(Source: The Great Jahy Will Not Be Defeated! Wiki)