Is the question about 'The Tyrant Falls in Love'? I'm familiar with that one, but not a title exactly called 'Tyrant Who Fell in Love'. If we're talking about the yaoi manga by Hinako Takanaga, then the main heroine concept is a bit tricky since the central romantic pair is male.
The story focuses on Morinaga and Souichi, with Morinaga being the obsessive 'tyrant' in love. There aren't any female leads in the traditional romantic heroine sense. The closest you might get is perhaps Souichi's sister, but she's a very minor character. The dynamics are all about the push-pull between the two guys.
I double-checked my shelves because the title phrasing threw me off. Maybe it's a fan translation variant? In any case, the core relationship leaves no room for a main heroine, which is part of what makes it stand out in the BL genre. It's relentlessly focused on that single, messed-up pairing.