u/Ga2fas

-The first time we read Subaru's story, he has already made a contract with Satella, his vow of silence and memory wipe are conditions.
-He has a strange attachment to Satella, saying he will save her despite never having formally met her.

Flugel and Subaru are the only two characters with an expert level knowledge of constellations. So what is their connection?

Flugel strikes me as a Subaru who never internalized that the people in this world weren't NPCs. He is not a good person. He makes Shaula wear a bikini for fun, makes her wait 400 years and orders her to kill anyone going near the Pleiades Watchtower. He is the Subaru who continued to view everything in the new world as a fulfillment fantasy. We already had a glimpse of this in the beginning of the story. People say that Flugel can't be Subaru and it's true, the Subaru we see has worked through his flaws.

Satella is obviously Emelia in the future. Both are half-elves, both have silver hair. Both are physically identical. When Subaru encounters Satella however speaks of Subaru with great love, as if they already have met, kissed, etc, which didn't happen until we read through the story ourselves. This seems to imply that at some point, Subaru will either somehow perma-die and Emelia will inherit Subaru's authorities, which includes the Authority of Sloth as seen with her many hands of Invisible Providence. She will somehow go back in time 400 years in the past, with all the incompatible Witch Factors and she will go insane, becoming Satella, the Witch of Envy. Just like how Petra became a Witch to save Subaru, Emilia would do the same. Satella is Emelia's future.

All the What-if stories are just echoes of a Subaru struggling to develop into a fuller human being.. Flugel is a stunted Subaru who got the exact Isekai escapist fantasy he initially wanted.

Satella having known the fully-realized Subaru we know, summons him into the world of Re:Zero. She bestows upon him the Authorities. Subaru gains incredible newfound abilities as well as the love of an yandere half-elf GF, and this newly traveled Subaru never goes through the character development of our Subaru. He abandons the name of Subaru, a shameful NEET boy from a shameful past. Flugel the Sage is his new name.

Flugel stands for "Wings" in German. A name fitting for someone flying away their past.

Satella gathers heroes to seal her when the time comes, as she knows she'll turn into the Witch of Envy. After a successful sealing, Flugal is unable to handle his lover Satella being basically gone, he travels 400 years in the future to where the story first takes place in order to save her. This is on the condition that he loses his memories and every Authority except for Envy. Flugel is gone and Subaru starts back to zero. Subaru loves Satella. Flugal loves Emelia.

Some further thoughts:
The question of how our Subaru will die is very clear, since there's only one possibility of someone who could permanently kill him. We will watch at some point Subaru be killed by Satella; jealous of Subaru's love for Emelia. Emelia will go back in time to.. "Save him."

Another realization I had: Why Satella has so many shadow hands: if Satella were to commit a murder-suicide at the end of the story, Emelia would inherit both Satella's and Subaru's (+1) Witch factors. Each new loop would add another one of failed Subaru's Invisible Providence hands to Satella's arsenal. There are over 2000 hands, meaning at least 2000 grand loops by the time we start the story. With enough loops, not just half the world, but everything will be destroyed.

TL/DR:
Flugel is Subaru's past (erased memories), a power fantasy fulfillment version of himself who was gifted the Authorites by Satella. We can count how many times the world has looped by counting Satella's shadow hands (>2000), which are each accumulated from a failed Subaru's Witch Factor.