A heartbreaker
A heartbreaker is the kind of person who takes your heart, crushes it with their bare hands, and then kicks it into the gutter. You're left with nothing but a broken heart and a bunch of questions.
They said they're dating someone else and didn't even ask you what you wanted.
You messaged them 20 times and they never replied.
They broke up with you and then started dating someone else the next day.