I'm unsure about spirits of the sun. Some mythical creatures associated with the sun are the dragons, and phoenix. One thing for sure that there are different lion turtles right? The fire lion turtle gives two different kind of elements, one is fire which the sun rules. Because of the heat, muggy air, and the Earth's core. There hasn't been any mention of sun spirit to tell the truth.Â
The only spirits of a planet would be the Moon spirits. It would be interesting if there's spirits of the elements, and maybe for the planets? Unlike the other ones like Mars, Venus, Mercury, Jupiter, and the outer planets.Â
Ooo I may have a good idea who could be the spirit of the sun. In the Legend of Korra Book Two, Raava is the Avatar spirit of light, and peace. This is good because without light there would be darkness. The sun is associated with Avatar spirit Raava after the last episode of Book Two, Raava will always be with Korra. Because she is light, peace, happiness, and good spirit. Her opposite counterpart is Vaatu the Avatar spirit of darkness, discord, malice, destruction, mortality, action, and vigor. I feel like he would be suiting for Avatar Spirit of Mars.Â
In Greek mythology, Ares is the god of destruction, war, malice, and discord. His Roman counterpart is also Mars. I'm unsure what would be the Chinese name for Mars.Â
Also Raava is saved but won't be able to interact with Korra. Because she almost got executed from Vaatu. Without light darkness will take over.Â