Come on dude! You had ONE SINGLE conversation with her before you died, and in it she told you that she wanted to take you to a love hotel! I'm not saying that she's not worth saving, but you clearly don't know the first thing about her! She straight out told you that she wanted to f*ck you, so stop blushing every time you see her/your tits!

According to Japanese mythology, Amaterasu and Tsukuyomi are both goddesses of the highest tier. They were also twins. Throughout the novel, the face of Tsukuyomi was always mysteriously covered up but she was known as the most beautiful goddess. Who else possesses unparalleled beauty? Lily of course! Lily also has a strange affinity with battle and is able to fight those above her realm with ease. As Yoritomo said: "Samurai train in order to fight, you fight in order to train" or something along the lines of that. Who else is a battle genius? Tsukuyomi of course! And since Tsukuyomi and Amaterasu look the same, its quite obvious who Senior Sister is.

At the end is is revealed that the DSM universe actually consists of a myriad of worlds separated by the "primal chaos" in which it is impossible for any entity to cross without their bodies being destroyed. Only when one has reached the realm of Omikami (The highest realm possible for a god), can they form a "true soul" which can cross the "primal chaos". But even then, the soul will become damaged thus losing memories. Lily was sent to Earth and was reborn in a boy's body, but due to her soul being female, it caused her body to become very frail. Despite losing her memories, her love towards Amaterasu did not wane. How did this happen? I will explain later.

The first main antagonist is Minamoto no Yoritomo. Big Dipper realm. He limited the cultivation in the East to Spirit Jade so that he could raise an army that could take over the entire mortal realm. It is revealed that he did not do this for power or greed, but rather to "save humanity" as the nights grow permanent. In order to "save humanity" he must seize absolute power over and "mutate" humanity into monsters so that they may survive the harsh environment. The duty to "save humanity" is passed onto Lily as she ascends into godhood to meet Amaterasu and request her to "bless the mortal realm with her light"

The second main antagonist is naturally Susanoo, Omikami of darkness, destruction of the sea (this does not seem to align properly with Japanese mythology). When Lily ascends to godhood and enters the heavenly palace to meet Amaterasu, she is instead met with a body identical to herself, but without a soul, just an empty shell. Susanoo had been trapped in the heavenly palace and is constantly trying to destroy the barrier of light surrounding Amaterasu's body. As the barrier grows weaker, Susanoo gains more [authority of the world] resulting in the days becoming shorter and the power of darkness spreading across the world.

She is, after all, the reincarnated goddess of the moon. As the "strongest battle goddess" her individual strength of the highest amongst the millions of gods, however, when faced with "betrayal" from her most trusted shadow, she was stabbed in the back and hence died. Susanoo, knowing that her true soul could never be damaged by "death"' forced her out of the 3 realms and into the primal chaos. When Amaterasu found out, she abandoned her body and followed after her.

In Lily's previous life, as Tsukuyomi she only loved Amaterasu. Before the 2 were born, the sun and moon simply did not exist. Amaterasu was the first to awaken her powers as the sun goddess and she protected Tsukuyomi as a elder sister figure. When Tsukuyomi awakened her powers as the moon goddess, it also meant that they could not be together except during solar eclipses that only happened a few times a year. This caused Tsukuyomi to grow exceedingly lonely even after raising the celestial army with various female generals who's love could never be reciprocated. She grew exceedingly cold and kept everyone at arms length. This ultimately lead to her downfall as Shimizu, previously knows simply as "shadow" caved in to Susanoo stabbed Tsukuyomi in the back (she has serious yandere tendencies)

Once she makes it back from Earth, she meets the reincarnations or descendants of her original generals (Uesugi, Shimizu, Ayaka etc.). As prophecy and destiny unite, Lily ascends to Takamagahara where senior sister awakens (vol 8). At this point her identity is not revealed yet but it was her that asked lily to "respond to the feelings of those who love her" as Amaterasu knew of Lily's loneliness in her previous life. She also takes Lily's v**ginity (a second time) (͡° ͜ʖ ͡°) and its like the author was freed from some metaphorical shackles allowing Lily to go all the way with everyone else (͡° ͜ʖ ͡°)

In the end she marries everyone as the bride, inviting only her closest associates in a modest yet heartwarming wedding. Instead of harem, it ends up with Lily being bottom (͡° ͜ʖ ͡°) to all the other girls, but she makes the resolution to turn the tables and eventually conquer (͡° ͜ʖ ͡°) everyone.

In the last battle between Susanoo and Lily, Lily was at a great disadvantaged as she had to invade the heavenly palace to rescue Amaterasu. Susanoo "devoured" thousands of lesser worlds including the billions of lives within allowing him to overpower the "strongest battle goddess". Lily used the ancient mirror to protect her spirit palace, which was revealed to the the ultimate defense tool allowing defecting of all attacks (to a certain power level). It shattered, causing Lily to use her hands to slow down Susanoo's sword. A shard landed near Amaterasu's body allowing her to regain the [authority of the world]. When Susanoo's sword was a millimeter from piercing Lily's true soul, Amaterasu transferred the [authority of the world] to Lily allowing her to exceed the limits of the 3 realms and overpower Susanoo.

To explain this how this [authority] works, we must first understand the concept of how the world itself is a form of power. The spirit power filling the air, the matter making up the world itself, the myriad of lifeforms and intelligence. Originally, Izanagi, the origin god held power of the entire 3 realms, but due to the betrayal of his other son's who wanted to seize power, the [authority of the world] was split between Izanami, Susanoo and Amaterasu. Izanami held the power of Yomi, Susanoo held the power of the seas of the mortal realm, while Amaterasu held the power of Takamagahara and the mortal realm. Tsukuyomi was a special case who held the power of "Yehara", the land where souls go after getting killed in Yomi. It was a special realm that was almost "outside the 3 realms".