The Four Saints (四聖人 Yon Seijin) are a group of characters from Fire Emblem: Three Houses.
Legend states that they fought alongside Seiros during the War of Heroes against the Fell King Nemesis over a thousand years ago. Together with the Ten Elites, the Four Saints are revered as the heroes of ancient Fódlan, the original holders of the Crests, and the founders of several noble bloodlines.
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In truth, only the Four Saints were allied with Seiros, while the Ten Elites were subordinates of Nemesis who participated in the slaughter of the Nabateans, the children of the Goddess, at Zanado. Seiros and the Four Saints were the sole survivors of the massacre, and orchestrated the War of Heroes against Nemesis to avenge their fallen people. As Seiros bestowed her blood upon Wilhelm Paul Hresvelg, endowing him with the Crest of Seiros, so too did Cichol, Cethleann, and Indech grant their Crests to other humans. Macuil may or may not have abstained from this act, as no living or dead human is confirmed to bear his crest (though Edelgard and Constance's B support implies that some noble line bore his crest in the empire before).
After the war, the Four Saints went their separate ways. Cichol and Cethleann assumed false identities over the centuries to protect themselves from those seeking the power of their blood, eventually becoming the "brother and sister" Seteth and Flayn before being invited to Garreg Mach Monastery by Archbishop Rhea. Indech and Macuil retreated to Lake Teutates and Sreng respectively, where they lived in isolation and guarded the weapons they had once used in battle. As time passed, all four lost their ability to shift between their human and bestial forms, with Cichol and Cethleann remaining human, and Indech and Macuil remaining magical beasts that came to be known as The Immovable and The Wind Caller.
As with Cichol and Cethleann, Saint Seiros also maintained a human persona called Rhea. However, unlike the Four Saints, Seiros was still able to shift between her human and bestial form, which was called The Immaculate One.