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|Full Name||Yotsuba Eisaku|
|Affiliation||Ten Master Clans (Yotsuba)|
|Novel||Volume 16, Chapter 2 (Mentioned)|
Yotsuba Eisaku (四葉 英作) was the second Head of the Yotsuba Family, which is one of the Ten Master Clans. He took over after the death of his older brother, Yotsuba Genzou whom past away during the war with Dahan in order to avenge his daughter Yotsuba Maya.
Appearance and Personality
Yotsuba Eisaku was born a combat magician like Tatsuya.
Eisaku analyzed how much magical power dwelled in Shiba Tatsuya when he was a baby. The truth of how much power Tatsuya had was made known to others because of how destructive it was. This spurred discussions when everyone got together to talk about whether or not Tatsuya should have been killed as a baby. Then Eisaku told the Yotsuba members to think about realistic responses than drowning in guilt and to reject proposal to kill Tatsuya. 
Eisaku pointed out to the others that they had inconsequentially obtained a force that could destroy the world. It was also a force, which could be the trump card of Yotsuba Clan. In due part because of this reason, Eisaku had Tatsuya raised and trained as a warrior as soon as he could stand up. 
During the training Tatsuya had his body optimized, for Eisaku was earnestly going to take advantage of him. 
Eisaku appointed his niece Maya as the next head when he was on his last few breaths. Before then, there hadn't been any decision about whether Yotsuba Miya or Maya would be made the next head. Maya was selected as the next family head per Eisaku’s will, it was not the result of family discussion. Yotsuba Maya continued the work of her uncle, to train Tatsuya, when she took over as head of the main house