Characters in this story are

Sun

Vulcan

Mercury

Venus

Earth

Luna/moon

Theia

Mars

Phobos

Deimos

Ceres

Jupiter

Ganymede

Europa

Callisto

Io

Elara

Amalthea

Saturn

Titan

Dione

Enceladus

Mimas

Rhea

Iapetus

Tethys

Hyperion

Uranus

Titania

Ariel

Umbriel

Miranda

Oberon

Neptune

Titron

Naiad

Thalassa

Despina

Galatea

Larissa

Hippocamp

Proteus

Pluto

Charon

Nix

Styx

Kerberes

Hydra

Haumea

Namaka

Hi'aka

Eris

Dysnomia 

Makemake

S/2015:136472:

Planet X

Ships:

Sun x Mercury

Theia x Mars

Luna x Titan

Jupiter x Saturn

Ganymede x Europa

Titania x Triton

One-sided: Titania x Titan

One-sided: Earth x Theia

Vulcan x Theia(They dated In the past)

That's its for the introduction