Eidolon Vessels
'Vessels' is the term used for the body copies that eidolons take from people. They may be taken due to personal aesthetic taste, or because the person in question has impressed the deity.
The original isn't taken over; their likeness is simply copied as a kind of empty shell, which the spirit/energy of the eidolon inhabits, and modifies to their liking. However, many if not all of the original people who have been used as vessels are long dead.
By using this method of transformation, the eidolons are much better able to roam and integrate with mortalkind. And some just enjoy having a smaller, simpler form to explore their world in.
Sylph for example, loves humans enough to walk as one, but is too shy to actually interact with them. So she often just spends her time flitting between forests, watching and blessing them from afar. She prefers a small, light and feminine figure.
Leviathan prefers to specifically become a very beautiful woman, usually dark and curvy. She's changed vessels the most often of her peers, but they always keep the same elements. She mainly spends her time living by the major ports, in the luxurious homes of wealthy sailors that she's wooed, and finds it fun to mess with their schedules by unsettling the sea when they're due to sail out.
Hekatonkheires (Hek) has kept the same vessel since the beginning, of an honest, kind and strong farmer, who worked some land not far from one of Hek's dormant volcanoes. Not only did the size and strength of the man appeal to him, but his constant hard-working nature and compassionate personality, despite living in difficulty, greatly impressed Hek as well. His own modifications mainly include tattoos hinting at his 100 arms covering his arms and torso.