Sly Marbo, the 'One Man Army', is a Catachan sniper who doesn't belong to a regiment. Feared by the enemies of the Imperium for sniping skills that would rival a Vindicare Assassin and survival skills greater than most Space Marines. 
As a Catachan native, the jungle is his element, and he is capable of taking on entire enemy forces through his innate skill at jungle and guerrilla warfare. He strikes from seemingly nowhere and disappears back into the dense jungle. He is famous for his excellent sniping skills and is considered to be one of the deadliest Humans in existence. 
Mysterious and aloof, little is known about Marbo's origins and most of what is said remains speculation. It is said that he was one of ten brothers inducted into the Catchan XII Regiment during fighting against Waaagh! Urgok on Ryza, where they were all killed. However, tales of a Guardsman returning two weeks later with the head of the Warboss began to surface. Since then, the myth of Guardsman Marbo has become legendary. On Pardus, it is said that he destroyed an enemy armored convoy by booby-trapping an entire ravine while on Bask's World he captured a command post single-handed, slaying the alien leader and all of its tentacled bodyguards. It is said that during the Octavius War Marbo hunted Lictors for sport. Marbo has a collection of medals that would rival a Warmaster, having been awarded the Star of Terra multiple times for his legendary exploits.
Both inside and outside of battle Guardsman Marbo is a loner who sticks to the darkness at all times, even when he is receiving orders from friendly forces. He is utterly sileny and acknowledges orders with but a slight nod before vanishing once more to find the enemy. Marbo is a patient hunter, and will wait in an ambush position for as long as it takes. He is the master of blending into his surroundings and covering any trace of his whereabouts so that the enemy has no idea from where the attack is coming from.
Marbo's characteristic weapon is the ripper pistol, a weapon firing toxic, armour-piercing rounds. For a melee weapon he carries the Envenomed Blade, a unique Catachan knife, the blade of which is coated in deadly venom.
Sly Marbo is originally based on the protagonist of the movie First Blood, John Rambo, Marbo is in fact the anagram of Rambo and Sly is the nickname of Sylvester Stallone, the actor who plays Rambo.