Rune was summoned by the half-demon Limizuki. After a war caused by Limi ravaged the planet's forests, she summoned a nature spirit to help replenish and care for it. In this universe, spirits do not take form until they are summoned, and when they are they often take aspects from their summoners- thus the reason Rune looks so much like Limi and not what you'd expect from a nature spirit.