When one of the first exchange-traded funds launched in 1993, securities lawyer Kathleen Moriarty received a gift from her legal assistant: a Spider-Man comic-book cover altered to depict the superhero facing off against a hulking Securities and Exchange Commission.

Twenty-three years later, Ms. Moriarty’s ability to navigate the arcane rules that govern financial markets and products has built her a reputation as a top lawyer in the ETF business and earned her the nickname “Spider Woman.”

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