Date Tomomune stands as the foundational ancestor of the Date clan, emerging during the late Heian to early Kamakura periods. Originally documented as Nakamura Hitachi-nyudo Nensai, he gained the territory of Date in Mutsu Province from Minamoto no Yoritomo in 1189 after fighting in the Battle of Ōshu. This pivotal event prompted his adoption of the name Date Tomomune and the establishment of the Date clan's stronghold at Takakogaoka Castle in modern-day Fukushima Prefecture, where his grave now rests in Koori.

Within the series, Tomomune manifests as a spectral ancestral figure summoned by the young Date Masamune through a mystical jewel gifted by a dragon deity. This artifact enables Masamune to invoke former Date leaders for guidance during his early trials as the clan's 17th lord. Tomomune focuses on delivering strategic counsel and wisdom, channeling his legacy as clan founder to reinforce leadership tenets and ancestral continuity. His presence embodies the Date lineage's collective insight, underscoring themes of duty and heritage.

Tomomune's leadership directly shaped the Date clan's cultural symbols, most notably the "Mibikiryo" crest—a circle enclosing three horizontal lines. Clan records attribute its origin to a curtain pattern bestowed by Minamoto no Yoritomo during Tomomune's era, cementing its status as the primary family emblem. His depiction consistently reflects his role as a guiding spirit, maintaining his historical and narrative significance.