Newcomers in armor storm up the field, humans encased in steel shells, wielding massive weapons.
PL 10/PP 150
Attributes: STR 16, DEX 12, CON 16, INT 12, WIS 12, CHA 10
Bluff 8+0 = 8
Climb 4+3 = 7
Disable Device 8+1 = 9
Intimidate 8+0 = 8
Knowledge [Tactics] 4+1 = 5
Notice 4+1 = 5
Profession [Marine] 8+1 = 9
Search 4+1 = 5
All-Out Attack, Attack Focus [Range] 1, Interpose, Move-By Action, Power Attack, Quick Draw [Load], Teamwork 1
Ablative Protection 7
Gauss Rifle 6 [Autofire 1 Penetrating Ranged Damage]
StimPack 6 [Personal Free-Action Boost [Gauss Rifle Autofire], Feedback: -1 Protection]
StimPack 2 [Personal Free-Action Boost [Speed], Side-Effect: Bruised]
Immunity 9 [Life Support]
Enhanced Impervious Fortitude 12
Extended Sight 1
Auditory Communications 3
Initiative/Defense: +1/18 
Saves: FORT +3 [+15], REF +6, WILL +6, TOUGH +10
Marines are the total package, all-around most balanced of the basic units. They have ranged damage at a wide dispersal, but more than that, they have the ability to become combat gods by harming themselves using the 'StimPack' devices, boosting their combat powers and speed to unholy levels.
The primary StimPack ability triggers a boost to their firing rate - represented by giving them a new attack power [the Boost 20 actually only ups them to damage 4 with their autofire guns, but that autofire allows them to do some horrible things to enemies]. The secondary StimPack power boosts their speed by 2, making them faster.
Marines also have the highest attack of all the basic units - +8 with ranged attacks, two higher than the Protoss Zealot's. However, to be fair, the Marines also utilize Autofire to a far greater degree.
Improving a Marine involves increasing skills, buying off the flaws in their powers, and adding new weaponry [perhaps transforming him into a Firebat?
EDIT: Already edited, the Marine attack bonus has been brought down, and their defense has been lowered [the shield entirely removed, the Defense has now been bought up directly].
EDIT 2: Boost has been brought into line, because I'm still not sure if that's the correct way to increase the damage on their Autofire, but that's how I'm playing it - 1 PP/1 PP, and this effect increases the Gauss Rifle's Autofire to 2 [by adding 6 PP to it, improving the Autofire a full rank].