From Halopedia, the Halo wiki
Childhood and Spartan training
He was born on the colony world of Harvest and was orphaned when his parents were killed in the First Battle of Harvest in 2525. After he was evacuated aboard a light freighter, he was sent to an orphanage where he lived until he volunteered for the Spartan-III project. He swore to get revenge on the Covenant for killing his family and destroying his home planet.
In 2532, Shane was sent to Camp Currahee on Onyx to be trained with Alpha Company by Lieutenant Kurt Ambrose and CPO Mendez. The first day there, he was pushed out of a Pelican by another SPARTAN-III recruit, Robert-A057, during a night-time Falcon Wing descent exercise. When he landed, he thought he had wet his pants, but in actuality, the skin on the back of his legs had been rubbed raw when he landed.  When they both landed on the ground, he and Robert began to fight, and they had to be broken up by CPO Mendez and a drill instructor, who Shane hit in the process.
Combat deployments and death
In late 2536, Shane graduated from Alpha Company, which was then deployed on numerous battlefields, against both Insurrectionists and the Covenant. These included the insurrection of Mamore, the Battle of New Constantinople, skirmishes in the Bonanza asteroid belt and the Far-gone colony platforms, as well as half a dozen other engagements over a span of nine months.
He was killed during Operation: PROMETHEUS, along with all 300 Spartans of Alpha Company on the same mission, by a plasma mortar that caused him to fly through the air and land face first, unmoving and lifeless. Shane had been the last Spartan on his team standing. Later, Kurt, together with Colonel Ackerson, Mendez, Vice Admiral Parangosky and Rear Admiral Ned Rich watched his last moments through a recording from an unknown Spartan's helmet camera. Kurt was profoundly distraught by Shane's death, along with those of the rest of Alpha Company. He attributed their demise primarily to loss of unit cohesion during the battle and later made an effort to emphasize the importance of teamwork when training the next batch of the Spartan-IIIs.
Shane was inspired to become a Spartan after seeing Kurt in his Mjolnir armor, which reminded Shane of the Green Knight from Arthurian legend.
List of appearances