Hi,
Personally, I would have preferred to avoid Katrin's part and focus more on the erotic adventures of the couple. However, my husband decided to create situations based on principles that reflect real life: love, passion, mistakes, revenge, fear, erotic fantasies, and so on.
Stefano's intent is to allow you to make decisions for your character without controlling the lives of others or skipping scenes involving these characters. Does it make sense? Perhaps. After all, Katrin already had a lover before MC found out.
I can only tell you with certainty that if you hate Brian, then Stefano has captured your attention. And I bet that, despite hating that character, you are curious to see if MC or Brian will prevail. Remember, this will depend on you and your choices.
A big hug from your friend, Luana.