Charles Ellicott Commentary 2 Samuel 12:3

"but the poor man had nothing, save one little ewe lamb, which he had bought and nourished up: and it grew up together with him, and with his children; it did eat of his own morsel, and drank of his own cup, and lay in his bosom, and was unto him as a daughter." — 2 Samuel 12:3 (ASV)

It grew up together. — “All these circumstances are exquisitely contrived to heighten the pity of the hearer for the oppressed, and his indignation against the oppressor.” — Speaker’s Commentary.