The Marine.
Marines are allies to the player and will actively kill any threats that are inherently hostile. Marines should they discover Xenomorphs will immediately focus fire on them and begin to try and wipe them out. Most times, this fails.
Marines can be very beneficial to the player, especially with the recruitment mechanic as it can make raiding hives easier due to have a back force to aid the battle. Marines are recruited by giving them a diamond.
Marines whilst not inherently hostile, will attack players should they attack them.
Marines can use any gun from the AvP : Humans module, and more if configured to do so. Whilst you typically wont see a basic marine with say, a rocket launcher you may see higher tier ones have stronger weaponry.
Marine Stats.
Marine Ranks.