Why Did Liara Become the Shadow Broker?

Liara T'Soni became the Shadow Broker because she spent two years hunting the old Broker, who had tried to sell Shepard's body to the Collectors and was holding her friend Feron prisoner. When she and Shepard finally killed him, she stepped into his place instead of letting his information network collapse, and promised to use it against the Reapers. (Mass Effect: Redemption; Mass Effect 2: Lair of the Shadow Broker)

Where the grudge started#

The story starts in the comic series Mass Effect: Redemption, set just after Shepard's death. Liara learns the Shadow Broker has agreed to sell Shepard's body to the Collectors. With Feron, a drell who claims to have left the Broker's service, she follows the body across Omega to the Broker's base. There she talks to the Broker through a hologram and asks why he'd hand Shepard to the Collectors. He tells her it's only business. (Mass Effect: Redemption)

Feron turns out to be a triple agent working for Cerberus. He still helps Liara get the body out, but he's caught in the escape and she has to leave him behind. Liara delivers Shepard to Cerberus and leaves with one goal: get Feron back and make the Broker pay. (Mass Effect: Redemption)

Two years on Illium#

When Shepard finds her in Mass Effect 2, Liara is an information broker on Illium, colder and more ruthless than the shy archaeologist from the first game. She threatens contacts, works the edges of the law and says little in her office because she assumes she's being recorded. The change comes from her obsession with the Broker. Shepard can help her catch one of his agents, the Observer, but she won't join the team. She has her own war. (Mass Effect 2)

Lair of the Shadow Broker#

In the 2010 add-on, Cerberus passes Shepard a lead on the Broker for Liara. When Shepard reaches her apartment, she's gone. The Spectre Tela Vasir had tried to shoot her, and Liara had left clues for Shepard before running. Vasir is working for the Broker. Shepard and Liara chase her across Illium, kill her and take her data, which leads them to the Broker's ship hidden in the storms of Hagalaz. (Mass Effect 2: Lair of the Shadow Broker)

On board they find Feron, alive but strapped into a torture machine, and then the Broker himself. He's a yahg, a species shut out of the galaxy since their homeworld was found. Liara works out that he must have started as the original Broker's captive, and the files on the ship confirm he rose through the organisation and killed his master. He attacks, and Shepard and Liara bring him down by turning his own barrier technology against him. (Mass Effect 2: Lair of the Shadow Broker)

Why she took the job#

Then the Broker's agents start calling, asking about the gap in transmissions. Liara sits at the console and answers them in the Broker's disguised voice, blaming a hardware upgrade. Feron arrives, gun drawn, and lowers it when he sees what she's doing. Both he and Shepard can ask whether this is what she really wants. She says it is. (Mass Effect 2: Lair of the Shadow Broker)

The game gives several reasons, and they fit together:

• Nobody would know. The few people who knew the Broker's identity were dead, so she could take over without a fight. (Mass Effect 2: Lair of the Shadow Broker)
• She'd seen what the network did in the wrong hands, and she promises Shepard she'll turn it around. (Mass Effect 2: Lair of the Shadow Broker)
• It's useful. Liara points out she can now give Shepard far more information, and she means to use the Broker's resources against the Reapers. (Mass Effect 2: Lair of the Shadow Broker)

She knows exactly how much power she now holds, and tells Shepard she could start a war within minutes if she wanted. It's fair to read her choice as both practical and personal: the thing she'd hated for two years becomes the tool she uses to protect the people she cares about. That last part is interpretation, but it matches how she uses the job. (Mass Effect 2: Lair of the Shadow Broker; Mass Effect 3)

The Shadow Broker in Mass Effect 3#

By Mass Effect 3 Liara is the Shadow Broker even if the player skipped the add-on. Without Shepard, she raided the ship with mercenaries and lost Feron. With Shepard's help, she and Feron later crashed the ship into a Cerberus cruiser rather than give it up, keeping the spy network, which she calls the part that really matters. (Mass Effect 3)

She moves the operation into Miranda's old office on the Normandy SR-2 and feeds the war a constant stream of intelligence, contacts and resources. She's also the one who tracked down the Prothean plans on Mars that became the Crucible. Only a handful of people know who the Broker really is. (Mass Effect 3)

Read more: Liara T'Soni, the Shadow Broker, and why Liara gave Shepard to Cerberus. (Mass Effect 2; Mass Effect 3)