
Hi! It was a long time ago that I read this novel, so I don’t remember the details, but I can answer your questions to some extent. If you really want to know the details, I might reread it.
Major spoiler alert!!!
As far as I remember, the first two volumes were really good, but the last volume got a little confusing. There were lots of time travelling going on, and Jade kept going back and forth in time.
1. Yes, Sage ends up with the blue king and lives happily ever after, or so it seems.
2. The person lying on the bed is Sage’s sister. (She died not long after they separated in the middle of volume one) She knew by dreams that there was no future where she didn't get killed, but out of all the possibilities, she chose the future where Sage could be with her when she died, and she didn't have to die alone. In the first volume, Sage couldn't talk with his sister before her death, but in the end of third volume, Sage goes back in time and stays with her until she dies.
a little bit more spoiler below!
this might spoil too much
1. The ‘Hisoku’ that Azure met when he was young (and probably had a crush on) and was searching for, was actually future Sage who went back in time.
2. After Hisoku's death, before future Sage goes back to the future, Azure sees him and calls out, 'Hisoku! ...or is it Sage?' At this point, Azure might have noticed that Sage might be Hisoku he was looking for.

thank you for the spoiler !!! unlike most people i really love knowing whats coming on ahead so please could you answer a question for me? this whole story how does gules play in to it? is he a character that makes likes one of the mc's or do one of the mc(sage ) like him? cause id really want to be prepared if im gonna encouter some angst and stuff!!!

Apparently this novel was written by an amateur writer and uploaded on website for her own enjoyment, which eventually got popular and became published as a book. Maybe this accounts for the story getting a bit complicated. I imagine she uploaded the story little by little daily or weekly. (There are lots of BL and 'light novels' that are published in this way. 'Re: zero' is one, for instance) Anyways, it's a good read.

Hi! I'm really sorry, I really need to reread the whole thing to answer your question, but there are bits and pieces of things I still remember about Gules.
spoiler below!
If my memory is right, Gules was attracted to Sage and Sage felt him a special friend, but Azure gets jealous and angry, and doesn't allow them to get close. He appeared a lot in the first volume but not so much in later volumes, which was really sad.

[SPOILERS: DON'T GET MAD AT ME LATER]------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Gules and Sage fall in love with each other!!! Like they're so precious together!!! Gules gives him a ring but the blue king suddenly appeared and Sage had to say rrreeeaaaalllyyy mean things to Gules. ┗( T﹏T )┛ughh I can't. I can't even imagine how much Gules was hurt. They should've fcking end togetheer!!! like whyyy?!?!! Gules don't deserve this. He's tooooo precious for this. ╥﹏╥ (/TДT)/ ┗( T﹏T )┛

Oh my thank you so much for telling me!! And I definitely won't get mad at you cause im really not good at handling suprises like I would be in a puddle of tears for day if I had gone into this without warning so thank you so much for this !!! Also like really I totally thought the blue king would be the uke's no. 1 from the beginning!!! So I have to say I'm really surprised!!

Have you seen the anime of 'Re: zero kara hajimeru isekai seikatsu' already?
If you can read Japanese, then here's the link.
http://ncode.syosetu.com/n2267be/
It's the very original version you can read for free.
There are lots of good novels by amateurs in the website, 'Shousetsuka ni narou' (it means 'Let's become a novelist').
The R18 BL only website within 'Shousetsuka ni narou' is called 'Moonlight novel.' I know of some novels that got published via 'Moonlight' too.

Sorry to disturb you again. I'm curious about one thing.
spoiler
In the novel, some pic show that Sage change his clother to more similar to the kings and he seem to have some strong power . I also realise the mark on his body change design and move to his hands instead of his chest. What happen to him, did he become a king too?

spoilers below!!
Sage saw a dream in which Hisoku was in danger, so he tells Azure where she might be. Hisoku was found to be the green king but she was already wounded and close to death. After her death, Sage becomes the green king. He develops a power of 'voice' -ability to convey words without uttering verbally, and to control people and phenomena to make what he said with his 'voice' to come true. This includes changing people's memory and making people to forget certain things.
Sage also traveled to past and present. This consumed lots of power. He also seemed not to have perfect control of this power, as he sometimes didn't know where and when he traveled to (something like fate led him to certain time and place).
I'll reread the books, but I hope there are some king people who could fill me in and give you better spoilers :) It may take one or two days to read, since I have some other things I want to read at present.

One more thing...
(spoilers)
Sex between two kings were said to be taboo, but it enhanced their power as king. (I don't remember clearly about this crucial part! lol) Anyways, you can pretty much predict what might happen from this setting. Sage needs lots of power to travel to the past to save people...
I saw the manga has 3 novels but sadly I can't read Japanese T_T. But I'm desperately to know the ending. Anyone kind soul here read the novel, please please help answer my questions regarding the novel? Thanks
spoiler alert.
spoiler:
1.Did Sage ended up with Azure the blue king and live a happy life?
2.There a last picture in volume 3, I think is the Sage/ Sage sister (I'm not sure but she/he wearing clother similar to the kings) holding someone weak hand while crying. That someone looks sick and laying on bed, but dont show his/her face. Anyone please help, what happen in that last picture? It looks not happy ending but I might wrong.
ps: sorry about my poor english.