The girl she saw Cyrus is Julianna/Yulianna (depends how you translate it) and is the daughter of the Duke and one of the CP possible fiancés.
FL wrongly believes Cyrus is in love with the Duke’s daughter, but this girl has “given” her some proof that they are in love (aka hickeys on her neck). Turna out, Cyrus hates Julianna’s guts and is only in love with FL.
“But he killed her!!!!” You will scream at me. He did not. It was Julianna (or her brother can’t remember 100%) transformed into Cyrus body.
Turns out Julianna is CRAZY about Cyrus and hates FL, wanting her to die so she can have ML. She tries, imprisoning her at some point, but Cyrus saves her.
Also, Cyrus remembers the first life too, that’s why he tries his best to hold himself back. That’s the reason of his glares, just him holding back the horny.
Read everything in a rush and was half asleep but that’s the gist of it 👍🏻
Man I like his yandere's vibe too much. His personality consisted of one part mostly in love and the rest is insane. And look at how forced he is, trying to look like he is normal and holding back his obsession. The artist is just too gooooood. Just my kind of red flag ❤️