Because she said "Why can't I fall in love....with such a good person ?" Ujioka was the only person who fully supported her and enjoyed her company, so she saying she only feels platonic love after all that ,, she shows she cares a lot about him like she faked loving him for a while and put full effort in giving him the dating experience he desired,Yeah 100% asexual and aromantic it's a strong possibility , why I can be so certain because I'm one and I asked myself that same question.also I've read the final chapter and know this isn't a phase or just she's not feeling it she literally says "That part of my heart that responds to such expectations, it seems I don't have it." when a coworker tries to get closer to her with romantic intent,she also ponders saying "Do I have to kill my soul just to fit into society?"so yeah she's aroace