Lyssa, other than being Trans, what's her angle?
Trans is a wildly popular genre/fetish among pervs, whether it's in regular porn or hentai, It would be a huge missed opportunity if they fail to include a trans character in the game that's called "Fetish Locator".
Now onto your question, besides being beautiful obviously, she had a somewhat mysterious aura around her when you first met her at the party which makes you want to learn more about her. Her giving you head initially only to end abruptly makes you want her even more, and the fact no matter what you say she will always deny you any more further pleasure only solidifies the impression she is the one in charge, compared to Dahlia's femdom scene for example, it's 10x times subtler and 100x times more alluring. It's not surprising to me that she is revealed to be a successful lawyer later, it just fits well from what we have seen from her.
All in all she struck me as a strong, independent and somewhat mysterious woman who knows what she wants and what she is doing. I am fairly certain you can remove her dick, or have an option to choose her to be a female at the beginning , she would still be a popular character among the players, she just has several character traits that many men crave for, even in real life. Her being a beautiful tall trans with a stunning figure only makes her stand out even more compared to the rest of the chicks.
At this point I am pretty sure she is at least one of the most popular if not the most popular character in the game, I mean just look at how many times "Lyssa" has been mentioned in this thread, you can't scroll down a few posts without seeing her name. It's a very well written character imo, especially by porn game standards.