A skilled mobile suit pilot, Sleg Sletcher was even described as the Union's top gun and living legend. In AD 2304, he was appointed by the Bell Factory to be the YMS-02 Union Blast's test pilot and received a handsome pay-check from the company. At that time, the Union Blast was competing with the Iris Corporation's YMS-01A Union Flag to become the next mainstay mobile suit for the Union. The Union Flag's test pilot was the young and unknown pilot, Graham Aker.
Sleg and Graham were once in the same military unit where they fought each other in 46 mock battles, with Sleg winning most of them. However, during the mock battle held as part of the competition to be Union's next mainstay mobile suit, Graham got the upper hand on Sleg. While Sleg stuck to the conventional aerial combat tactics of the time, Graham innovated with his 'Graham Maneuver' (aka 'Graham Special'), which he created prior to the battle and allowed his Union Flag to transform during flight. As the mock battle came to a close, Sleg attempted to crash his Union Blast into Graham's Union Flag in a desperate attempt to win. However, Graham countered by speedily transforming his Union Flag into Mobile Suit Mode and then using his machine's Sonic Blade to slice off one of the Union Blast's wings. The Union Blast then crashed into the ground, killing Sleg.
Many thought that with Sleg's superb piloting skills, he should have been able to stabilize his machine and land without issues. At the same time, there were rumors that Sleg was facing financial difficulties and had insured himself for a high price. However, these rumors were never confirmed. Due to Sleg's death, Graham earned the infamous moniker, "Superior Officer Killer".