Kogoeru is Shimo's master/sensei she trained her how to use breath of the ice she had her fair share of similarities to Shimo she was also the Ice Hashira and she was Shiro's sensei too she made Shiro and Shimo's sword Kogoeru dies in the end because demon slayers thinking she was a demon but is was Shimo who was a demon Kogoeru was already sick but was still killed