After a run of more than 250 days, I finally got the 'handsome boyfriend ending'.
There are two stages:
1. In the first stage, the end goal is as follows: when the bf proposes going to a love hotel, Tomie's 'Cherish me a little more!' dialogue option needs to succeed. Prior to this, Tomie shouldn't have acted too much like a slut (so don't reply seductively but friendly, wear sexy clothing but not the last clothing option). Also, maybe the 'act charming' skill helps in this stage, but I'm not sure. In any case, keep attractiveness and pheromones above 300, elegance above 80 (and a high reputation doesn't hurt as well). Eventually, as said before, the bf will suggest to go to a love hotel and then you need to pick the 'cherish me a little more' option. If this succeeds, you'll go to the second stage.
2. In the second stage, the goal is to move Tomie's relationship further after having had sex. Tomie's bf will bring her to restaurants and buy presents for her. What's important to note is that, even in this stage, Tomie should keep wearing sexy clothing (and not the sluttiest option) and keep answering in a friendly rather than a seductive manner. You'll notice that you're on the right path if (1) Tomie remarks that her bf sure seems to love her when you select the option to call Tomie's bf; (2) after sex, Tomie says that she can feel him hugging her warmly. Eventually, after sex, Tomie can ask for a more serious relationship.
An important thing to note is that it's imperative for Tomie to have (1) >300 attractiveness and pheromones; (2) >80 elegance. Even then, it's quite a grind to actually receive the ending. Since it doesn't have H-scenes in its ending, I'd only recommend going for this ending if you're a completionist.
