Kenji Harima and Eri Sawachika

A plot in the series is that one day while Harima was down and out on the street mistook the hand held out to him for Tenma's. Taking a major leap of bravery, Harima confessed everything that was in his heart, only to see that the girl who he had just confessed to being in love with was Eri, one of Tenma's best friends. It then becomes a running gag that it'd Harima and Eri were a couple, or Eri's annoyance at Harima over other misunderstandings would cause her to be a tsundere-type of character. Since Harima is a former delinquent and Eri comes from a wealthy family, they are total opposites of each other in terms of social standing. As the series goes on, it becomes more and more evident that Eri has feelings for Harima. For his part, while Harima admits Eri is attractive, not only was he still in love with Tenma, but also considered Eri his mortal enemy.