Complex sentence: As a child, having lost his mother at a young age, Uryū mostly interacted and trained with his grandfather Sōken Ishida before witnessing him being killed by Hollows right in front of him.
Split into simpler sentences: As a child, Uryū mostly interacted and trained with his grandfather, Sōken Ishida. At a fairly young age, Uryū's grandfather was killed by hollows right in front of him.