The Strategy Told from the Grave (Bohyou ga Oshieta Sakusen- 墓標が教えた作戦) - S1.E3

When fighting the Boazanian, Ippei accidently damaged Dr. Mitsuyo's grave post. There's surely a clash between Ippei and Kenichi over that incident. Ippei's past was then revealed. Ippei thought her mother abandoned him when they were in America so he hated her and all mothers. Later on, Ippei learnt that her mother died when trying to protect him and his sick father when they were attacked by wolves.