A love rival is fine, it can force the characters to take action. As long as it stays as just a rival and doesn't swerve hard into haremshit territory.
@BCS In actuality, the age of consent in Japan is not a uniform 18, or that bullshit 12/14/whatever age. Different prefectures add obscenity crimes for sexual activities with a minor on top of the statutory minimum age, making an effectively older or younger age of consent than 18.
In Tokyo, this rounds out to an AoC of 20, not 18.
So is she in love with him or she just want to screw around with his feelings? Because she mostly looks like wanting to wreck their relationship than actually loving him