Mastema "hostility" is the name of an arch-demon and chief of the demons engendered by the fallen Watcher/Angels with women, perhaps one of those same demons.
His actions and name indicate he is Satan, the 'Adversary', but much more the Satan who appears in the book of Job with a function to fulfill under God than the Satan of later tradition who is the uttermost enemy of God. Beliar, mentioned twice in Jubilees, is likely to be identical with Mastema in this work.
When God is ready to destroy all these demons after the flood and Noah prays that his descendants be released from their attacks, Mastema intervenes, beseeching God to allow him to retain and control one tenth of these demons in order to exercise his authority because they are "intended to corrupt and lead astray before my judgement because the evil of the sons of men is great". So Mastema is the tester of humans with God's permission.
Mastema sends a plague of birds onto the land in the days of Terah.
Later, Mastema counsels God to test Abraham (Jubilees 17:15-16) just as Satan in the book of Job wants permission to test Job. As Abraham prepares to sacrifice his son Isaac, Mastema stands in God's presence. On his deathbed Isaac promises that the spirits of Mastema will have no power to turn Jacob or his descendants away from Yahweh.
The strange account in Exodus 4.24 where Yahweh meets Moses by the way and tries to kill him is retold in a way that attributes the attack to Mastema instead (Jubilees 48:1-3). It is claimed that Mastema aided the Egyptian priests that opposed Moses. Mastema is also said to have been chained while the Israelites left Egypt but then let go to encourage the Egyptians to chase after the Israelites and so come to their doom in the Red Sea.
On the other hand, the deaths of the firstborn of the Egyptians are attributed to "all the powers of Mastema".