Lu (Thrice curse her name) got hit with this odd feeling sometimes referred to as "guilt" after The Incident, and patched him back together.
Or maybe she just wanted something to do with her hands while her next vessel finished cooking.
Or maybe he's one of two perfectly ordinary war orphans, saved by the Heterodyne Boys and left in Beetleburg to be raised by the perfectly ordinary Clays.
He'd tell you the latter.
Whatever dark and twisted spark craze gave him his life back left marks, most blatantly the stitches, but also various pigmentation abnormalities, a slight stiffness in movement, and near-debilitating headaches.
Regardless, none of that quite managed to prevent him from being a somewhat nosy and startlingly well-reasoned child, his foster sister being able to remember many times where he confronted their guardians over something they were trying to keep from the kids. (Not what any of said things were, though. She didn't realise the situation was important back then.)
After a devastating workshop accident when he was barely 20 necessitated a second revival, KB immediately broke through as a practically unprecedentedly powerful spark, specialising in psychosocial analysis.
The last anyone's heard of the eldest Clay child is that he left to be a Wulfenbach spark after an explosive (though not quite literally) fight with his adoptive parents, said parents allegedly burning all letters received from CW since.
