Light Yagami/Harry Potter
The real challenge for Light's influence over Dudley, and consequent emotional torture to Vernon and Petunia, started a year later.
After a full year when Petunia and Vernon tried all manner of threats, bribes and even an attempt of corporeal punishment to separate Dudley from Light, Dudley turned four. Soon after, the youngest Dursley started pre-Kindergarten.
Light, as usual, wasn't given anything that the Durlsey's had to spend money on. Pre-KG wasn't for Light. Vernon had sneered when he had said that. Truly unfortunate. Not. Light was already a university graduate, from To-Oh, an internationally ranked university for law at that. He had no interest in sharing classroom space with children who didn't even know how to spell their own name.
Hours and hours the kid spent away from Light with real children and made friends. That was still okay. As long as Light acted as he did with Misa, asked "How was your day, Diddykins?" and leant a half-hearted ear to Dudley's commentary, he was content.
Then two months into pre-KG and Dudley changed. He grew easily frustrated, angrier over how much better he was over Light and that his cousin should "act less weird, you freak, you don't know anything". Now that. That.
Light had tried. he had tried so, so hard that the apple would far from the tree. But then this!