Well theres two things ive heard keys do in a metal band.
(#1)The keys lay down a solid backbone as the bass does.The phrase stays simple and sets the mood.
(#2)The keys do a more complex melody along with the guitar/s,usually in an interlude.
(#3)The keys can also fly off with or without the guitar/s and solo.
Both techniques are often used together in a band/song.

eh...well I guess ive heard three things.
Listen to some Soilwork and/or Children Of Bodom to get an idea of the role keys have in a metal band.

Remember though...nothing is written in stone.Experiment and have fun.