Icarus was born one of a set of triplets born to Lady Ksenia of House Essair (Nee Warwicke). They were not her first children, but were the first children of her marriage to Vopiscus. To his knowledge, Icarus is the second-child - his brother Kassandros is the oldest, and Cygnus was born shortly after he was.
It was perhaps surprising that in a set of triplets all three of them survived infancy. The eldest was not quite like the other two. He hit milestones much faster than they did, and generally seemed older than them, developmentally - consistently. It was one of those things that, for the most part, did eventually even out. Even so, he was always special. He was always powerful - and of course, he was always the first born. Always the one who would follow Vopiscus as head of house.
Kassandros was an unsettling child who always seemed older than he was. Icarus was not so in the same way, but he was, in his own way, strange. He was the sort of child that learned to entertain himself young. Cygnus always needed attention, and in some ways, Icarus shifted to try to take less of that away from him.
They were six when their mother left. Some people said it was because of Kassandros and how special he was. But as Icarus got older, he decided that if she'd really been a good mother she'd have stayed, no matter how hard it was to handle Kassandros and his power. He wonders, if she'd been a decent person, if she'd raised them with love instead of walking away... he wonders if maybe their family wouldn't be as broken as it is. If maybe Kassandros would be more of a person instead of a walking powerhouse that has almost forgotten that he is Hume too.
Icarus supposes it didn't matter. Because she didn't love them enough to stay, and so he'll never know what life might have been like if she had. At any rate, during their childhood years Icarus started doing magic - his particular affinity is ice.
Icarus' attempts at being invisible so Cygnus could have more of the spotlight weren't really effective. And in the end, Cygnus sought the attention he wanted elsewhere. It was a shame, really. But he always did hope that his brother found good friends that understood him better. On the other hand, in watching the whirlwind of his brother, Icarus was always afraid that he would fall in with the wrong sort.
Icarus ended up going into the military as well, somewhat against Kassandros' will. It is the only thing he's ever really fought about Kassandros about, usually being content to obey his brother's orders. Kass was stronger and smarter anyway - and Icarus always knew that. Some part of him wanted to grow into his own and that was why he went into the military. At the same time, Kassandros wants to be Imperator, and the easiest way to do that is for all of them to bring glory to the Essair name.
He doesn't think Kass can do it by himself. More appropriately, he doesn't think he should. But, Icarus is just the middle child, and neither of his siblings really listens to what he thinks - and he suspects neither exactly cares. Still, he does generally have their best interests at heart, as much as he can.