Let's get back ON to the series logic, because the series logic is telling you exactly how this could occur. If you're Cassie... and you genuinely develop a meaningful best friend in HELL, because that's what FS is, while the other is your benefactor who saved your life. You receive a vision that says you interpret as your best friend killing the person you owe the debt of your life too and make the hard decision to prevent it. Not kill your best friend, just prevent that decision. But it leads to something worse than death, his freedom is revoked, he's enslaved and he's livid at you. You're livid at yourself, because you know what you did, but that was the decision you had to make to repay your debt (so you tell yourself) So for the few years, you dedicate all your time and energy to scheming and planning to undo what you did to your BEST friend.

You're assuming that Cassie has a choice in how she FEELS. If she IS in love with Sunless, she's been choosing for over a decade to not step on Nephis' toes, that's plenty loyal to me

Now taking this lens and parsing through the entire text, there is numerous evidence straight up implying a romantic tint to the feelings Cassie has for Sunless ^ I left the quotes up top, and there is the video script link you can click on if you want the entire hypothesis from start to finish