I haven't explored it too much, but Colonel Henley, Weddle, and Hamilton all went to Sanglamore with Sigil's older brother James. The four of them were all posted to the 3rd Lyran Guard just before the War of '39 broke out. James is critically wounded in an engagement with the 5th Sword of Light. He swears then Leutnant Jason Hamilton to watch out for his brother Sigil during the upcoming war. Ultimately, Henley gets Sigil transferred to the 3rd Lyran Guard and assigned to his lance. And thats how it all begins...
Sigil was trained as 'Mech technician not a MechWarrior. He never studied tactics or strategy. By all rights he should have been an engineer, but his desire to strike back at the Kuritans for the death of his brother made him want to climb into the cockpit. He actually considers himself a TechWarrior.
I might need to develop this further, because your right, without it, the Colonel would have kicked Sigil out of the unit long ago... :o