The offence under Section 137(1)(b) of the Bharatiya Nyaya Sanhita, 2023 is basically against whom?

Section 137(1)(b) of the Bharatiya Nyaya Sanhita, 2023 (formerly section 361 IPC) defines kidnapping from lawful guardianship as taking or enticing a child, or a person of unsound mind, out of the keeping of the "lawful guardian" without the guardian's consent. The offence is essentially against the guardian's right of custody, which is why the child's own consent is irrelevant; it is not framed as an offence against the family, society or any person generally.

A child here means a person below eighteen, regardless of sex; the old IPC limits were below sixteen for males and below eighteen for females.