There's basically just two things I don't like about Estefania. First, she's a forced LI with a prior history with the MC that we just get to see in bits and pieces of reminiscences. With most of the other LIs, we're starting more or less from scratch and get to see how the relationship develops from start to finish. The other thing is I disliked the scene where she slaps the MC -- yeah, it somewhat fits given her passionate nature and quick temper, but it seemed like something out of an old sitcom. I'd prefer not to see domestic violence be played for laughs or be depicted in a lighthearted way.
Other than that, she's a great character. She clearly loves the MC more than anyone else (including Fernanda at this point). She sticks with him even if he isn't treating her particularly well like when he sends her out for technology lessons alone with that one creep. I found her openness with her emotions to make for a good contrast with the ubiquitous tsundere love interests in so many VNs who treat MCs like garbage because of their inability to express their true feelings. Whether Estefania is feeling happy, sad, loving, or angry in a certain moment, you will know it, and I found that refreshing. She's also the most physically attractive character in the game in my opinion.
