 If you liked
Death Note
|
 ...then you might like
Sugoi Smartphone
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Super Smartphone and Death Note are two Shounen Jump manga where the main character is a highly intelligent high school student who seems to be bored with his life when he gets a mysterious item with impossible abilities, with someone there to guide the main character into using it. Some people have even started calling Super Smartphone "Death Note 2.0", since MC uses it for what seems to be for the benefit of others and there's a lot of inner narration to spell out his deductions. Nothing crazy dark has happened yet unlike DN, though.