After years of criticizing regulators for lax enforcement of securities law, Elizabeth Warren finally has an outspoken ally on the inside pushing for a crackdown on Wall Street offenders.

Kara Stein, the junior Democrat on the Securities and Exchange Commission, has quickly become one of its more ruthless enforcers, making trouble for Wall Street, and aligning herself with Warren’s views as part of a powerful regulator that polices the financial industry.

Stein has opposed giving free passes, or waivers, to badly behaving financial companies so that they can keep doing certain types of business as if nothing happened. Before she started dissenting, the waiver approval was often a routine formality at the SEC.

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Though she may not have the Massachusetts senator’s star power or political ambition, Stein has rattled financial companies, several banking lobbyists said.

“If you have a motion for a waiver pending before the SEC, Stein is…