I retract what I had previously said when covering Season 3 given I was adamant that Subaru and Petelgeuse’s souls had not fused and that Sirius was delusional in uttering that. I had tunnel vision about one of my previous theories and didn’t see what Tappei was trying to tell in that moment. I now believe Sirius was speaking the truth when she said that Subaru and Petelgeuse’s souls amalgamated/fused, and I also believe Sirius, who happens to have elven ears, silver hair, and purple eyes, isn’t Fortuna but also an amalgamation of Fortuna and a spirit, which would allow Petelgeuse and Fortuna to parallel each other in how they continued to exist after they were supposed to die, and this would explain Sirius’s obsession with Petelgeuse given Fortuna said that she came back to ensure that she and Petelgeuse either both lived or that they died together due to her not being able to live on without him visiting her.
Why Subaru and Petelgeuse’s souls likely amalgamated into one is that Sirius felt Petelgeuse’s presence when Subaru used Invisible Providence in Episode 53. Regulus was under the impression that Sirius thought that Petelgeuse had taken control of Subaru much like he did with his Fingers, but that was likely not correct given Sirius said in Episode 66 that Subaru and Petelgeuse’s souls had amalgamated into one, showing she never thought that it was what Regulus said in the first place.
This is backed by how Emilia in Episode 62 thought to herself that that’s where Geuse was all along after Subaru used Invisible Providence to have a shadow hand phase through Emilia’s chest, and only then was Emilia able to sense Petelgeuse. Emilia may have subconsciously recognized Petelgeuse in Episode 23 of Season 1 after she defeated him when he had possessed one of his Fingers, but Emilia has never known Unseen Hand to be able to phase through objects, so she shouldn’t have been able to intuit that it was Petelgeuse just by the hand phasing through, and thus I conclude that what she sensed was the genuine Petelgeuse.
Now on Sirius, I believe Sirius isn’t Fortuna but also an amalgamation of Fortuna and a spirit, which would allow Petelgeuse and Fortuna to parallel each other in how they both continued to live on despite what should have been their deaths.
I think one of the spirits who guided Emilia to the Seal may have matured into a full-fledged spirit and felt strong emotions from Petelgeuse and Fortuna’s feelings for each other, fusing with Fortuna’s soul in the process with the hope that their love would continue. Pandora may have planted the seed in a spirit similarly to how she asked Regulus if he felt nothing from seeing Petelgeuse and Fortuna. The spirit may have been emotionally affected in a positive way whereas it sent Regulus into a flying rage since he had come to make Fortuna his wife and was incensed to see that she already loved someone else.
We know spirits that aren’t full-fledged can ensure that they don’t physically materialize, so you never know when there might be a spirit nearby. What we do know is that there were spirits when Pandora and Regulus arrived, and there were spirits when they brought Emilia to the Seal, so it’s possible one spirit vibed very strongly with Petelgeuse and Fortuna’s love and tried to ensure that it continued only for it to be all fucked up ’cause this is Tappei’s story.
Given Fortuna indicated she’d have nothing to look forward to if Petelgeuse stopped coming to the forest, and that’s why she came back to ensure that they either both lived or that they died together, Petelgeuse probably featured very heavily in Fortuna’s final thoughts, and said spirit I propose was there may have desired so much for Fortuna and Petelgeuse to be together that it amalgamated with Fortuna’s soul, hence becoming the entity we came to know as Sirius. This would be how she knows Subaru and Petelgeuse’s souls fused ’cause it literally happened to her, and she was speaking from a position of experience.