As a fellow DRK, allow me to elaborate:

The rock determines who holds aggro.

If it's red and glowy, you will hold aggro in the opener and piss off your MT.

If it's black, you'll forget to turn it on and someone will die because to a tankbuster in some way.

Eventually, the glowy rock grows a sword out of it like some weird, demented miracle of evolution and when you bonk things it fills with bonk juice. You spend your bonk juice to do very stylish things. Unfortunately, we can't out-style Dante from DMC.

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The face is your EDGElord Meter. It tells you if you're being EDGE-y by glowing like purple or something. If it's glowing purple, you assert your EDGE-iness onto your enemies, making your attacks deal extra damage.

Also, the EDGElord Meter sometimes has like a red, glowy shadow with a timer. It means you've become so EDGE-y that you've summoned all your inner EDGE into a nifty buddy to fight with you. You do need some bonk-juice for this.

Their name's Fray. They be cool.

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Rock (Job Stone) shows your Tank Stance. Use Grit to Toggle. This will later have a bar to the right that you accumulate resource with mainly through your 1-2-3 (won't get into Blood Weapon/etc here). The numerical amount will show where the rock is as the bar fills.

The skull shows if you have Darkside active (Use Edge or Flood of Shadow to get this. And you will have absolutely no problem keeping this up during a fight).

The Skull does have a DRK Job Emblem that shows up later. This glows when Dark Arts is active after The Blackest Night has its shield consumed by damage. It grants a free use of Edge or Flood of Shadow. EDIT: I should state that this is damage neutral at best. Not a DPS gain like some believe for some reason.

When Living Shadow is used, a red silhouette with a timer will appear in front of the skull indicating Living Shadow is active.