Well I am bored right now, and since Carter was too lazy to make one, I'll make one.
So this is pretty much the total opposite of the overrated characters thread (NO SHIT!).
I can think of a couple characters, who come to mind:
Matsuda (Death Note): Sure this guy was an idiot, but he doesn't seem to get enough credit with the fans. He was a definite help at the end of the series.
Aizawa (Death note): I have always liked this guy, and a lot of people really haven't noticed. He really helped towards the end of the series also.
Kakyoin (Jojo's bizarre adventure): Once again, he really doesn't seem to get noticed by anyone, but was a huge help in the end of part 3.
Enel: A lot of people only think of him as an overpowered person with an unbeatable devil fruit (except in Luffy's case of course), but he was a pretty brilliant fighter.
Sensui: Man this guy was awesome, and barely gets any credit.
So yeah, Who do you think is underrated?