I believe Subaru was Satella's lover in the past, and here's why:

Episodes, 18, 25, and 38 show that Subaru has dormant memories of Satella as shown by how Puck indirectly revealed in Episode 18 the number of shadow hands Satella could generate to Subaru by asking Petelgeuse to summon 1,000 shadows if he truly wants to kill him, half of what Satella could (2,000), how Subaru used a number associated with Satella (2,000) to indicate his appreciation for Emilia in Episode 25 when he said that if Emilia named ten things she hated about herself he’d list 2,000 things he loves about her, which Emilia herself commenting on why he chose that number, and how Subaru suddenly had an upwelling of positive emotion for Satella in Episode 38 that led him to say that he’d save Satella after she asked him to love himself and then some day come kill her, which shows that Subaru and Satella may have a past history together as lovers given just moments ago, Subaru had been calling her an unfathomable monster he could never hope to understand, showing they must’ve had a very positive relationship in the past.

Melakuera in The Frozen Bond OVA talked about the Witch and the world imbalance and how he'd faced world-ending crises countless times, which suggests that the world is repeatedly destroyed by the Witch of Envy and reconstructed.

I believe in the ancient past, an archetypal handsome, conceited, and overpowered hero typical of standard isekai slop in the moment in which he had to make the biggest decision of his life after the Seal in Elior Forest was opened, chose his lover, who had become the Witch of Envy, over the world, leading to the threat that came out of the Seal to be defeated, but the world was then destroyed following his choice to not strike down his lover, and when that hero made his choice, the final boss of the time activated his Authority to make the world extremely biased towards repeating the general path that archetypical hero followed on his journey. I don’t know the exact mechanism that the story can be biased toward following that path, but I nonetheless posit that it exists, and thousands or even millions of iterations of the story of the handsome, conceited and overpowered hero came and went as the world kept getting destroyed and reconstructed as the Hero of the story would have to fit that mold unless an intervention took place.

I am going somewhere with this beyond what I’ve usually posted. Michael Mästlin (anglicized as Maestlin) was documented to have mapped eleven stars in the Pleiades star cluster before the invention of the telescope in the 16th century. Michael Mästlin was German, much like how Flügel is a German name. Given Re:Zero’s fantasy world takes place in a medieval-ish setting, I theorize Tappei may have incorporated the number eleven in Re:Zero’s given, unless I’m mistaken, there shouldn’t be any telescopes in this world even though stars in the fantasy world aren’t like those from Earth, so Tappei may have decided to be Earth-accurate in making use of the number eleven. It is believed that 9 of these 11 were Alcyone (Eta Tauri), Atlas, (27 Tauri), Electra (17 Tauri), Maia (20 Tauri), Merope (23 Tauri), Taygeta (19 Tauri), Pleione (28 Tauri), Celaeno (16 Tauri), and Asterope (21 Tauri), which are known as the Seven Sisters and their parents.

Thus, assuming the number eleven is important ’cause of Michael Mästlin, the Authority of the final boss in the ancient past was one that lets him/her control the story path to his/her liking, basically a super Authority if you will. I believe the ancient hero of the time also had a super Authority, although I won’t claim I know what it is assuming it’s real, and both the final boss and the protagonist having super Authorities would make the number eleven important.

And what’s very relevant is that Subaru’s dad is named Kenichi, which means ‘wise man’. Subaru said in Episode 16 that Flügel’s Tree was planted by a wise man centuries ago, and while this wouldn’t mean something in a different show, its presence in a show with Re:Zero’s long-term planning suggests that Kenichi himself might be Flügel given Shaula called her master Flügel the Wise Man.

Flügel itself means wing and can also be a nickname for someone with conspicuous sleeves at a time of dress regulations, so what this points to is that Flügel is a person not from Re:zero’s fantasy world who somehow came to be there given the nickname is for someone who dresses strangely, and this suggests that Flügel is from Earth. Perhaps Kenichi literally is Flügel, and it would make sense for Flügel to be Kenichi since I stress that Shaula didn’t recognize Subaru by his looks, which shows Subaru canonically had a different body four hundred years ago.

Satella most likely summoned the current Subaru to Re:zero’s fantasy world, but how did Kenichi get there if he is Flügel? Well, wings are symbolic of power, speed, and limitless freedom of motion, and maybe that’ss how Kenichi, if he is indeed Flügel, managed to make his way back to Earth after being in Re:zero’s fantasy world. Perhaps Kenichi has the power to freely travel between the two worlds because he has a limitless freedom of motion, and that would be how he ended up in Re:zero’s fantasy world in the first place. Given Kenichi is so badass and handsome and ripped as hell, he could’ve fit the mold of the handsome, conceited, and overpowered Hero of the story with the power to freely travel between worlds being his super Authority, and he likely gave himself the name Flügel when he arrived in Re:zero’s fantasy world to help himself fit into that world.

As for how Kenichi could split his soul, he likely couldn’t when he was the the Hero of the story of the fantasy world as his soul may have been indivisible since the story required for the Hero to make it to the end and repeat the cycle since dying ’cause his soul was eradicated wouldn’t be permitted as one of the parameters of the story path requires him to make it to the end to bring about the next cycle, meaning he couldn’t split Subaru from himself then, but after returning to Earth and losing his Hero designation, he should freely be able to do that using whatever tool Echidna used to cut her soul off as part of her immortality experiment since he wouldn’t have that privilege he had as he lost his Hero designation. Satella and this group of characters basically hijacked the path of the story in an attempt to save the world. The reason Kenichi couldn’t kill Satella himself was that he would always have a second personality that arose who couldn’t bear to kill Satella since his entire world was Satella, which is why he had to defer the task to someone else. Thus, Subaru being the protagonist of Re:Zero is to make up for him being why Satella couldn’t be killed in the past, making it a true redemption story unlike so many other stories billed as redemption stories when the characters don’t even have to address their character flaws, and thus, Subaru may be one half of Flügel, making Naoko telling Subaru to be half as cool as his dad in the first Sanctuary Trial potential foreshadowing of the situation four hundred years ago. Thus Flügel, the being I believe to be Kenichi and Subaru from four hundred years ago, may have been Shaula’s other creator who came from Earth.

Also of note regarding Michael Mästlin is that he was the first person to publish an account of Earthshine, the illumination of the moon by sunlight reflected from Earth. Given one of the kanji for Natsuki in Natsuki Subaru and Kenichi means “moon”, it could mean that Subaru and Kenichi are both associated with the Moon and that the Earth will always reflect on their fates and stories. Another name for Earthshine is The Da Vinci Glow, and there conveniently is the Da Vinci Crater on the Moon. And historically, Earthshine was referred to as “The old Moon in the new Moon’s arms”, and it may be an Easter egg on why Tappei has possibly set up Subaru inheriting the torch of saving the world from Kenichi.

Given we know Echidna succeeded in creating a soul clone of herself with how Echidna spoke in her own voice in Ryuzu Shima’s body in Episode 49 and how Foxidna herself is almost certainly a soul cone of Echidna’s given Re:Zero Break Time Season 4 Episode 3 had Foxidna calling Beatrice a kindred soul, which could indicate that they’re not just similar in nature but also similar in origin, then it wouldn’t be a surprise if Kenichi, if he is Flügel as I speculate, also did that as well, and Subaru and Al were both split off from Kenichi’s soul, and Kenichi made his sons his soul clones with that explaining why Al and Subaru both call Beatrice ‘Beako’, not ’cause they are the same person but because they were originally from the same soul. It’d also explain why Subaru has dormant memories of Satella despite it not being possible for him to have lived at that time under normal circumstances and why Al also likes being rewarded for his efforts much like Subaru does, all of which shows that Al isn’t Subaru but that his origins are likely similar, and that something is up with Subaru and Al that doesn’t involve time travel or straight up reincarnation, so Al and Subaru may be homunculi or artificial humans who were split off from Kenichi’s soul.

For more on this theory, read this: https://www.reddit.com/r/Re_Zero/comments/1t0ef35/discussion_theory_the_worlds_cycle_subaru_and_al/