Okay, so this is kinda scary. It's understandable why Millie wanted to be closer to Zen despite her being a little cold. They have been in similar positions as the bearer of expectations in the academic aspect. But because they are the same age and, unlike her mother, Zen has never broken a promise to Millie in the short while they've known each other, she's gotten the comfortable relationship she was looking for. Someone she can throw her selfish feelings to without being scared.
There's also the fact that it's reciprocal in a sense. Zen's worldview is also expanding thanks to Millie's unrelenting advances. I'm expecting more insight in Zen's family too, as it seems it will be an important turning point later on.
For now, Millie is definitely on her way to falling in love first. Will Zen still be oblivious I wonder.
Edit: Grammar mistakes. I swear I got to work on simplifying my writing. Too many subordinate sentences.