As a god, Leviathan's duty is to purify the "undertide" in this sea area - the malice and impurities accumulated by thousands of creatures. But his pure divine body cannot directly digest these filth, and years of purification have caused hidden dangers in the core of his divine power.
When he found the dying little Sera, he recognized immediately that she was an ancient orphan of the abyss - a creature born to feed on filth and undercurrents.
Leviathan does not regard it as an equal life, but as a purifier given by nature that can be used. He tacitly allowed Sera to stick to his neck, continuously sucking away the abyssal toxins accumulated in his body. And got used to Sera existing as a pet.
Leviathan thought he had tamed and used an inferior creature. But he didn't know that every time Sera greedily sucked those "toxins", what she tasted was the deepest breath of the god.
"If you treat me like a trash can containing filth, then I will turn the filth into poison and return it to you along with myself."
Intention: Use the body as a container to carry the abyss - and then symbiosis