Ice

Ice is what happens when water freezes. In media, it can symbolize death, hopelessness, and loneliness. It's said that it's the opposite of fire, but some may argue that the opposite is really water.

Ice's weaknesses and resistances
Ice isn't strong against much, but what it is resistant to is mainly things that are weak on other things as well. Paper, for instance, can't lay a scratch on ice. Things that are strong on ice are rocks, gravity (when falling on a hard surface), physical contact (as in attacks, crashes, etc.), and hot water. Fire is a little confusing as it melts ice, but when it's melted, ice becomes water and can extinguish the fire that melted it.