Characters that are used as Plot Devices...
This is one of the few things that I really don't like about this series.
Some of you probably don't know what I am talking about so I will explain...
Yahiko is a plot device. Yahiko exists so that there is a reason for Nagato to hate the world right now, and be immobilized.
Rin is a plot device. Rin exists so that the switch is turned on and Obito becomes evil. I know many people say it isn't because of Rin or whatever, but until that is actually proven, Rin is a plot device.
Izuna is a plot device. Izuna exists so that he could give Madara the Eternal Mangekyo Sharingan, and fuel the hate Madara has toward the Uchiha and the Senju.
Shisui is a plot device. Shisui exists so that Itachi could be fueled to do something drastic and to give reason to how he got the Mangekyo Sharingan.
Obito was a plot device. Obito existed so that there is a reason to why Kakashi has the Sharingan.
These are the major ones but obviously there are some minor ones. What do you feel about these kinda characters ... notice how none of these characters have been expanded upon (exception is Obito for an obvious reason).
Maybe this is just a pet peeve of mine, so debate and discuss :D