"koro" is the time span such as "while the sun shines so harshly", "during the last summer", "recent days (chikagoro)". I hope this helps

ころ　＝　One span of time means season, period and age

日语好像很难学啊

I think think it means "When". Kodomo no koro ni akogareta Hero. =Hero that I had a yearning for when I was a child. ”koro” is "ころ" and "頃" in Japanese.