Although his father Yoshikage was, at the time of his birth, 22 years old and had two elder sons, Yasumori was known as the heir of the Adachi family as he had since his childhood called himself 'Kuro,' the name only allowed for the heir.
父義景は22歳、2人の兄がいるが、泰盛は当初から安達氏嫡子の呼び名である「九郎」を名乗っており、安達家の跡継ぎとして周知されていた。