Roughly 500 years ago, before Signora's recruitment into the Fatui, she was a maiden and was called as Rosalyne and she then eventually fell into a mutual love with Knight of Favonius knight, Rostam. Before the calamity, Rostam gave Rosalyne a special hydro-timepiece in order to keep on tracking the amount of time that she would be studying at Sumeru Academia. Presumbly given before Rosalyne goes abroad to Sumeru Academia. Albeit, once she returned, as she heard about Rostam's death, she was never to be the maiden who loved to sing ever again.
In the Defender's Will artifacts, some of the artifacts talks about Rostam and Rosalyne.
In the Guardian's Flower artifact, it mentions that the flower was an iris who Rostam cherished, unfortunately, the chance was said to never come. Due to his death, he was never able to give Rosalyne the iris. And in its description, "Only that maiden's clear voice, Could cause the guardian's brow to come unbound.". Stating that Rosalyne was the only one who can cause Rostam's brow to come unbound because of her voice. "Their time together was short. At last, the knight's blood would run dry, As would her tears and her songs.". Stating about Rostam's death and how their time together was only short.
In the Guardian's Clock artifact, it talks about of his friend and his superior having their future with no fear. "Only when he gazed on that maiden in the square, Could he think upon things he had no time for.". Until he saw Rosalyne in the square, he then thought about things that he had no time for.
In the Guardian's Band artifact, it talks how Rostam wore the band, "As long as he was sufficiently prepared, Even if one day, he should be unable to defend this land, Or protect his friend, or that maiden who loved to sing...". And talking about how as long he was sufficiently prepared, then he can protect the maiden (Rosalyne,) who loved to sing.
In the Elegy for the End weapon, in its lore, it mentions about a certain song that make Rostam cause his brows to unfurl, and only a certain girl on the square (Rosalyne), could cause him to forget the great burden of his duty for a time. During the calamity, as Rostam fell, about to die. He wishes to hear Rosalyne's singing again, however she was aboard studying. And lastly, the lore mentions that Rosalyne changed once she returned, and after her voice and tears ran dried, she swore to user her life's flames to cleanse the world's crookedness. In which case, how she became the Crimson Witch of Embers.
Rostalyne is currently small within the Genshin Impact as their lore and relation can only be explored in the Defender's Will artifact. And there isn't that much yet known about the two due to Signora's death. However, a lot people portraits them having an angsty interaction. Due to the fact Rostam died, and eventually Signora died as well. There are about 17 works on AO3.