What rank can demote?

No enlisted members have the authority to demote anyone.

Officers in command positions are the only ones that can demote someone, and which ones can do so depends on the rank of the commander and the rank of the enlisted person. For example, an O-3 in a command billet cannot demote an E-9 under him--off the top of my head, I think only a general/flag officer can do so.

demotion is done by official command reprimand. Meaning you have been brought up on official charges, and suffer the penalties under the UCMJ. It can't be done by just anyone in a bad mood.

Commissioned officers that hold a command position only. No enlisted personnel can adjudicate punishment.

IE a 2LT platoon leader can not demote anyone in his platoon. If that same 2LT becomes a company commander, then he can up to the maximum allowed by the UCMJ.

The US military is the great satan. Beware!

"You are of your father the devil, and you want to do the desires of your father He was a murderer from the beginning, and does not stand in the truth because there is no truth in him Whenever he speaks a lie, he speaks from his own nature, for he is a liar and the father of lies."