Hong Kong’s Security Bureau has defended its decision to appoint singer Hins Cheung as a mentor in a “rehabilitation project” for youngsters arrested during the 2019 protests, saying it welcomes individuals from all backgrounds to share positive life experiences following criticism from internet users in mainland China.

The project, which has been running for two years, aims to support the rehabilitation of about 7,000 people arrested during the social unrest but not prosecuted.

“[Activities]...