All of them are skeletons and are sanses. Each of them are from a different timeline where events played out differently. Tears lost his brother (Papyrus) and is completely depressed about it, Call of the Void (aka Cov) made a deal with gaster to help take down the human with a DT extractor, Betrayer is a sans who was betrayed by the human. They have a back up sans if things goes out bad (Karma) unlike the other sanses Karmas left eye turns purple instead of blue and can use Act/ Glitch things 