One of Steven A Cohen's must-trusted portfolio managers is planning to strike out on his own, according to people familiar with the matter, the highest-profile departure from the hedge-fund firm since it was tarnished by an insider-trading scandal.

Gabriel Plotkin, a portfolio manager who focuses on consumer stocks and has managed more than $1 billion in positions for Cohen's SAC Capital Advisors in recent years, will leave by the end of the year to start his own firm, the people said.