Stagnate and Gracefeel are in fact, sister goddesses. Both used to manage the cycle of things beyond death, but Stagnate got mad that perfectly good people died and wanted to change the status quo into an undying utopia... since heaven doesn’t exist (you reincarnate instead).
Long story short, creating an “Echo” meant Stagnate possessed a person or thing somehow, and some “distortion is necessary”, changing her personality slightly. Newborn Echoes tend to be infantile, emotional, and rash, and she blames her bad behavior of their first meeting(s) on that.
She’s still eager to have him survive and become a true hero, so she gives him some advice (a divine revelation) in book 3-1, and Gracefeel surprisingly agrees with what Stagnate says (basically: “The advice is right, but don’t approach her too much.”
In volume 3-2, she outright states she’s a woman (because she was presumable using gender neutral pronouns in Japanese or something), and would love to carry William’s child whenever she’s able to create her next Echo.