The Cherethites are mentioned ten times in the Old Testament. (The NIV uses the spelling Kerethite, and one time the KJV uses the term Cherethims while using Chrerethites the other times.)
Based on 1 Samuel 30:14, “We had made a raid against the Negeb of the Cherethites and against that which belongs to Judah and against the Negeb of Caleb, and we burned Ziklag with fire” (ESV), it seems that Cherethites is another name for the Philistines. It could also be that the Cherethites were a sub-group of Philistines or a people closely associated with the Philistines. Ezekiel 25:16 and Zephaniah 2:5 make the same connection.
The other times that the Cherethites are mentioned are in association with King David. There were people from the surrounding lands who were loyal to David, and he had squads of soldiers/mercenaries who appear to have been organized by nationality. For instance, 2 Samuel 15:18 tells us, “And all his servants passed by him, and all the Cherethites, and all the Pelethites, and all the six hundred Gittites who had followed him from Gath, passed on before the king” (ESV).
So, the Cherethites were a defined group within or closely associated with the Philistines. The Old Testament does not intend to say that all of the Cherethites were loyal to David, but that there were some Cherethites who were loyal to him. Beyond that, the Old Testament simply does not give any more information on the Cherethites.