Monday's only known appearance was from the very first teaser trailer. Only his name was visible and no outline number was shown. Monday was meant to be the squad leader who betrayed the United State Colonial Marines. This was axed when the squad gameplay was scrapped.
It is very possible that Monday was going to let Weyland-Yutani PMCs onboard the Sephora and then disable the systems inside the Sulaco. It is unknown if he would have survived in the original campaign mode had the Tactical Gameplay/Squad Roles remained.