Seiwa Genji
Clan
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The Seiwa Genji is a line of the Japanese Minamoto clan that is descended from Emperor Seiwa, which is the most successful and powerful line of the clan. Wikipedia
Final ruler: Minamoto no Sanetomo
Founder: Sons/grandsons of Emperor Seiwa, e.g. Minamoto no Tsunemoto (源 経基)