in the HomeWorld Series, Emily Hawthorn's father Sherman Hawthorn creates a monster named Leviathan. Gaia , the co-founder of Hell Colony , fuses himself with the monster and takes control over it. from that time forward the monster is known as Gaia, although Gaia is not the creature, but a an controlling the creature. thus, when Gaia is defeated by Emily Scott-Harris , She does not defeat the monster, she merely takes control of the Monster Leviathan and casts out the spirit of Gaia. Leviathan itself is a very powerful creature and is not evil. She can perhaps do good things in the form of Leviathan, and probably would do so, but her father would still mourn her even though she is alive in this creature.
The body of Leviathan is indestructable, and made partly of Living Stone so it can only be controlled by a Chosen One and can only be safe if controlled by a perfectly pure spirit. Thus it is better for Emily to stay with him at least until another Chosen One becomes pure enough to take over that role.