Not really an affair but basically the Phantom (mask dude) chose to teach the other girl more than the pink girlie.
The other girl called him ugly even though he was teaching her the perfection of ballet.
That made pinky angry at the other girl for being ungrateful and chosen by the one she "loves"
For the Phantom to choose the other girl tho- it might be intentionally an open ending why he was happy seeing the other girl get hurt.
Seeing them talking about having the Phantom obsessed with pinky, it might mean that dude is happy that pinky is obsessed with him too for her to do that.
Basically they match each other's freak. （￣へ￣）
Great interpretation, I also interpreted it that way.
It's like the Phantom purposely teach the other girl to awaken the pinky girls hidden side or it's like the switch or the last step so that the pinky girl would be obsessed with Phantom guy.
Since in the ending the Phantom guy was happy, it's like he achieved his goal, which you've said is to make the pinky get obsessed with him the way his obsessed with the pinky girl.
So basically there are two characters
Hwaran, which is the role the pink haired lady is playing, and Giselle which is a role inside a role. (Basically hwaran as a ballerina is going to play Giselle)
Giselle in the ballet, abandons everything for her lover Albericht who turns out to have been engaged to another woman.
While Hwaran in the phantom of the ballet film, also tells the phantom that she wants to fall in love, the phantom is against it and wants to stop her from being distracted from anything but ballet (and himself) so he picks a junior and starts training her. Which makes Hwaran jealous.
In this part, Hwaran gets fully immersed in her role as Giselle and starts confusing her reality with the story she is playing. She starts making an analogy by comparing the junior (from her real world) and the rival fiancee woman from Giselle. Proof to this is that when she decides to hurt her junior, she says : I'll get rid of (the fiancee woman's name). And just like what Giselle wanted most was Albericht, what Hwaran wanted most was the phantom.
This is why ths phantom is happy after Hwaran hurts the junior because it basically means that she got rid of her "rival" just to get him. And now he can have what he wants, which is Hwaran as a ballerina and her full attention on him.
wait im confused at the last part of the ballerina the ballerina kill the girl bc she said smt bad about the mask guy ( i forgot the names lmfao) or she killed her bc she was the affair woman in the plot