Yu Mong-in(柳夢寅, 1559~1623) is one of numerous intellectuals against dethronement of queen and killing brother(廢母殺弟) by the Dae-buk(大北) faction during Reign of King Kwanghae-gun(光海君). At the same time, he is the sole intellectual adhere to the Fidelity to his Principle(節義) for King Kwanghae-gun(光海君). Yu Mong-in's these political posture is What for? To answer this, I trace the six months process from Coup of the king In-Jo(仁祖反正, 1623.3) to Yu Mong-in's dying day(1623.8). And I examine the historical and literary tradition and the meaning of the Fidelity to his Principle(節義) for King Kwanghae-gun(光海君).