Board Thread:General Discussion/@comment-33063265-20170910144543/@comment-33994734-20171217233141

So to answer your question, it's wildely accepted that Subaru was in that world at the same time as Satella even if not as Flugel.

Then becoming Satella's first and only Love.

As Subaru is a sage candidate and Shaula recognizes him as Flugel (she has poor vision so it's no confirmation at all) it's also wildely thought he IS Flugel. Few other alternatives can be risen as of now, the only alternative I can see being Pack (other theories have people being split into multiple people so it's also possible Subaru and Pack might be parts of Flugel. It's just wild guessing for now though.).

Adressing another point from your post:

Satella loved Subaru from at the very least the day of his (last) summoning, as shown in the dream castle chapters. It's only at the end of these dream castle chapters that she asks that he comes and kill her, to which he replies he will save her. She however couldn't reply to this as the dream world faded at that time. So answering "I'll definitely save you" was not a factor in that Love.