A common theory from fans is that Fortuna is Sirius Romanee-Conti given Sirius in the Season 3 anime outside of the Break Time shorts has silver hair, elven ears, and purple eyes and loves Petelgeuse. Personally, I don’t subscribe to this theory since in the Re:Zero fandom it’s very common for fans to come up with the X character is EXACTLY Y character theory, whether it involves time travel or not, which I believe is most likely a result of many fans either not having read much fiction or reading poorly-written works.
I believe Sirius is the result of an impressionable lower-level spirit (lesser spirit or whatever the spirit level of Julius’s spirits are given they directly follow Julius’s instructions) that was in awe of Petelgeuse and Fortuna’s love given Pandora told Regulus how he can look at them and feel nothing, which was likely her taunting him to bait him into killing her ’cause she knew Regulus has deep trauma over being a poor husband with the mere thought of Fortuna’s and Petelgeuse’s love reminding him of his personal failure. I believe she also said that for the lower-level spirits to hear, and one of them was strongly moved by Pandora’s words.
Thus, when Fortuna died, one of the lower-level spirits perhaps matured and fused with Fortuna’s soul, and that’s who Sirius is. Given in Re:Zero Break Time Ep. 79 Sirius said that her hometown was destroyed due to a witch hunt with witch hunts becoming common during that era, and she said that misfortune follows her wherever she goes, Sirius may have been the target of persecution repeatedly and repeatedly attacked for her silver-coloured hair and purple eyes despite her not being a half-elf.
And that’s probably part of the reason Sirius’s Authority manifested in an ability that lets her herd people into feeling the same thing since she’s seen so much hatred, and as a low-level spirit before she became Sirius, it may have thought that love is two people feeling the same thing since it felt Petelgeuse and Fortuna’s love for each other, leading to it wanting their love to continue after Fortuna died, and that’s the second reason for her Authority of Wrath allowing for her to herd people into feeling the same emotion.
And so, Sirius’s biggest regret could be fusing with Fortuna’s soul in the first place ’cause doing so only led to her experiencing pain and suffering due to her past persecution and ’cause of Petelgeuse not reciprocating her love after she fused with Fortuna’s soul, which is why she asked Priscilla if she understood her pain, so if that’s indeed Sirius’s biggest regret, assuming my theory about Subaru’s Authorities being shaped in part by those in the dark world has any truth to it, one of Subaru’s Authority of Wrath abilities may be to split apart souls that have amalgamated with each other.
Now to share some relevant background info, for Ep. 79, I posted a theory that Subaru’s Authorities being shaped in part by those in the dark world (Relevant scenes of the dark world being when Subaru remembered being in the dark world with his heart caressed by Satella in Ep. 8 after Puck used Shamak on him, when Satella whispered, “Aishiteru” to Subaru during the White Whale fight in Ep. 20, and when Subaru saw both Petelgeuse and Regulus before seeing Satella in Ep. 70).
For example, I believe Petelgeuse deeply regretting having killed Fortuna is probably why Subaru gets recoil from using Invisible Providence offensively since that act hurt Petelgeuse so much and drove him insane after whatever Pandora told him to help him cope with what he’d done, and we know it did given Petelgeuse was talking about how Subaru killed Rem in Arc 3 with his (Petelgeuse’s) own hands with tears in his eyes, showing that the moment in which Petelgeuse killed Fortuna with his Authority hurt him to his core, and that’s what prompted him to project his biggest failing onto Subaru. This would be why Subaru wasn’t hurt when he merely entered Invisible Providence into Emilia’s body but was pained when he crushed Regulus’s heart and why he wasn’t pained when he rubbed Joseph’s neck with Invisible Providence.
Petelgeuse even killed Rem in the brutal fashion he did likely ’cause he deep down remembered how he killed Fortuna and since trauma survivors like to put themselves in situations that re-traumatize them, that’s why he was crying in that scene since he subconsciously tried to draw a connection between a man killing a woman he loved, mirroring what had happened to him since he saw Rem’s overwhelming love for Subaru, much like how Fortuna came back to Petelgeuse in the past ’cause she couldn’t take it for him to die all on his own, leaving her behind.
Much like I propose for Regulus Corneas with his remarks about how others are violating his rights and how he says that he never needed his wives to smile, which I believe were coping mechanisms due to avoid dealing with his negative self-worth centered around having failed his first wife, who he’d actually cared about, which is backed by his rambling to himself in Season 3, demonstrating that he had serious trauma over his wife smiling in her final moments even though by self-admission no matter what he’d done before that, she’d refused to smile, showing that her not smiling was not for a lack of trying on his part, Sirius justifies her actions of “spreading love” to avoid dealing with her negative self-esteem, meaning this coping mechanism has also allowed her to stay functional or alive up until this point as I believe was likely the case for Regulus up until Season 3. I believe on some level Regulus and Sirius know what they’ve been doing is wrong, but they stay in denial to avoid dealing with issues core to their being.
If you recall in Episode 13 of Re:Zero in Season 1, Emilia called Puck a Great Spirit of Fire, which shows that ice attacks can be formed through temperature manipulation in Re:Zero. Priscilla’s flames in Ep. 61 based on Yang magic are bright white, which shows that more than one type of magic can produce flames, and this suggests that it’s possible that Sirius’s black/purple flames are based on Yin magic. That’d make the Fortuna and Sirius situation an inverse of the Satella and Emilia situation with Satella’s natural magic affinity being Yin given the mass of shadows and the shadow hands which can be seen by all aren’t the Authority of Sloth while Emilia’s is Fire, which would be a nice contrast to how one who fell from grace in Sirius ended up having the Yin attribute, an attribute associated with darkness, while Emilia doing her best to live despite the prejudice she faces has the Fire attribute with fire often representing life.
My reasoning for this being a good story development would be that individuality and identity are core issues in Re:Zero with the exploration of twin issues in Arc 2 with Rem’s inferiority complex centered around Ram, of Emilia not being Satella in the Frozen Bond OVA, of Subaru being his own person rather than a knock-off of Roswaal, of Subaru having space to grow out of Kenichi’s shadow, and the Ryuzu clone stuff in Season 2/Arc 4, and of amnesiac Subaru being treated as his own person in Season 4/Arc 6 as well as whatever they’re doing with Reid overpowering Roy. I believe aside from the topic of growing to love oneself, there’s nothing more consistent in Re:Zero than the topic of individuality/identity, so a spirit amalgamating with someone else could be explored in the future with both Sirius and Subaru if I’m right about the defeated Sin Archbishops merging with Subaru’s soul. It’d be in line with how Tappei has written his story with further exploration of the topic of individuality and identity as the story progresses.