Shining Light
Bored beyond comprehension, Ryuk decides after seeing what humans do with the Death Note, to experiment with what would happen if a human lived their whole life with access and knowledge of the Death Note. This leads him to find four-month-old Light Yagami, whom he decides to use as his test subject and has a little fun.
Light's parents, as with anybody else, can't see Ryuk and thus think Light is possessed, so while trying to find answers, Soichiro turns to a young cadet for assistance taking care of his son.
Light grows up knowing he is different but rarely bored with Ryuk by his side throughout his whole life and having little interest in using the Death Note, training in Shinigami magic and learning the ways of the world instead. But things become interesting when, after multiple incidents regarding his secret, someone else with the Death Note begins using it as the mass killer known as Kira.
Add in a dumbfounded detective and a task force all aware of Light's situation and far past caring, and Ryuk knows he made the right choice because he's found plenty of entertainment, even though he has to fight the protective instinct he's developed towards Light.