Ella Irwin, the Twitter executive in charge of content moderation and policy, has resigned, leaving new owner Elon Musk without one of his most loyal lieutenants.

As head of Trust and Safety, Irwin helped oversee Twitter’s policies on harassment, hate speech and violent content — rules which were relaxed under Musk. She was also heavily involved in account suspensions, and would frequently respond to Twitter users whose accounts were blocked.

Irwin confirmed her departure, which was earlier reported by Fortune, in an email.

Current and former employees have said Irwin was one of Musk’s most trusted executives, willing to carry out and defend his decisions on content as the team’s efforts became strained with layoffs and firings. While that work improved her standing at Twitter, it also alienated advertisers and users who felt the platform had become more hospitable to hateful content.

When Musk took over, advertisers began fleeing Twitter because they were concerned about the type of content their ads would appear next to. Since October, Twitter’s advertising revenue has declined by 50%, Musk said in March.

Irwin is the second head of Trust and Safety to resign since Musk took over. The first, Yoel Roth, left in November. After his departure, Roth was a vocal critic of the company.

The reasons for Irwin’s resignation are not clear. Irwin joined Twitter last June, a few months before Musk’s $44 billion acquisition closed.

(Updates with Irwin confirmation in third paragraph)