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!

LILY WAS NEVER MALE IN THE FIRST PLACE HAHA. Don't want to spoil her true identity although you all may have guessed it already, but the body she resides in was always her own, and her soul was always female, it was just temporarily molded into a cute shouta for certain reasons. A good gender bended is no gender bender at all. Lily also marries all the love interests at the end of volume 17, best scene ever, made me want to cry. Lots of yuri s*x scenes later on, pretty sure it was in volume 12 where Lily loses her v*rginity to Senior Sister, and Lily wanted to have her first time with Senior Sister, the author decided that since that happened, Lily could go do it with all the other love interests as well. Whenever Rei and Shimizu (i think that was her name) pushed Lily down, she no longer had a reason to refuse.

As many of you have guessed, Lily's true identity is Tsukuyomi. And obviously senior sister is Amaterasu. In Japanese lore, Tsukuyomi and Amaterasu were twins, however Amaterasu was male. Obviously, the author did a little switcheroo and since they are female twins, they also look identical (at least in the physical sense). Now, how was Lily turned into a shota you ask? It would be a very long and complicated story so I'm gonna keep it short. Basically, in the 3 realms, there are (very few) Omikami who hold the power of the word itself. Before Tsukuyomi and Amaterasu were born, Izanami and Izanagi (their parents) held the entire power of the word split between the 2. One day, another on of their children (don't remember who, as technically all the gods are their children) tried to steal that power by betraying and ambushing Izanami and Izanagi. Ok I don't exactly remember exactly how it happened but it ended with Izanami ruling the underworld, Tsukuyomi ruling the "land of the night" which is separate from the 3 realms and the place where people go after dying in the underworld. Amaterasu ruling heaven aka Takamagara, and Susanoo ruling the human realm's oceans. The oceans are very large btw which is still considerable power. It was described that Takamagahara and Yomi (underworld) was millions of times larger than the land of the mortal realm, however the mortal realm is the same size of Takamagahara. So almost the entire mortal realm is just ocean. Obviously susanoo didn't like ruling over basically nothing, so he decided to try defeat Tsukuyomi and Amaterasu. Basically some stuff happened and Tsukuyomi was banished out of the 3 realms into the primal chaos, in which she drifted for 1000 years purely in her soul state to modern day Earth where she lived for 20 years (ish) as a boy. As her soul was a woman, the boys looks turns out to be a cute shota (lmao). Obviously, when Amaterasu found out that Tsukuyomi was banished, she left her body in the Heavenly Palace under a heavy protective spell and left the 3 realms in her soul form to find Lily. Over the 1000 years, Susanoo used all his efforts to try break the protection on Amaterasu's body, hence explaining why the Sun was fading, whilst after finding Tsukuyomi, they teleport back to the 3 realms (in a process that I'm not gonna explain, however it was explained very well in the novel surprisingly). Amaterasu ends up using all her soul power to do this, while Lily wakes up having no idea what's going on and under the misconception that she "stole" Senior Sister's body.

Btw Lily and senior sister aka Tsukuyomi and Amaterasu were in fact married before, but they could only meet during solar eclipses or whatever they were called. Kinda sad, but this was resolved at the end of volume 17 when Lily takes the power of the entire 3 realms for herself, becoming a being that transcends the 3 realms and truly becoming untouchable. Amaterasu still holds the power of the concept of the sun, but they are no longer forced to be apart.