The woman whose quirky, eclectic personal style came to define J. Crew — and who is the No.2 executive at the company — is leaving.

Jenna Lyons, president and executive creative director, is stepping down from the struggling retailer. She will remain with J.Crew as a creative advisor until her contract expires in December 2017.

Lyons, a 26-year J. Crew veteran, has been in her current position since 2012, and held various design positions prior to that. She started to take on a higher profile after Millard “Mickey” Drexler arrived as CEO in 2003, becoming a style icon in fashion circles with her signature geek-chic look, complete with oversized black glasses.

Lyons’ departure comes as J.Crew struggles with weak sales in its namesake brand and a heavy debt load. Total company revenues decreased 3% to $2.4 billion in 2016; J.Crew brand sales were down 6% with a 5% decline in same-store sales.

The retailer said it does not plan…