Slattern

Slattern is a Category V Kaiju, making her the largest and deadliest to ever come out of the inter dimensional breach. She is the secondary in Pacific Rim, and was introduced as the main antagonist, but this was a misinformation campaign made to hide the film's true main villains.

Pacific Rim

With Leatherback and Otachi's loss, the Precursors hasten Slattern's creation and send Raiju and Scunner to guard the breach.

Slattern emerges from the breach when the two Jaegers Striker Eureka and Gipsy Danger approach the rift itself in a mission to collapse it with a thermonuclear bomb. Just as Striker Eureka gets to the edge of the Breach, Slattern appears and quickly overpowers Striker by attacking the Jaeger with its multiple tails, sending it flying into a wall and badly damaging it.

As Striker Eureka recovers from the hit, Slattern leaps on the Jaeger and tackles it, using the rocky surroundings to cause further damage. However, as the two giants continue to battle, Striker gains the upper hand on Slattern, creating a slash across the Kaiju's throat and wounding it in the chest with its arm blades. When Slattern realizes it's losing, it calls out to Scunner for assistance. As Slattern and Scunner begin to converge on Striker for the killing blow however, its two pilots, Stacker Pentacost and Chuck Hansen, decide to detonate their nuclear payload in order to clear a way for Gipsy to enter the breach and destroy it (as Gipsy Danger is powered by a nuclear reactor and this could serve as a substitute bomb). The resulting nuclear explosion kills Scunner and destroys Striker Eureka, killing its pilots. However, Slattern miraculously survives, though severely wounded.

Just as Gipsy is about to make the leap into the breach, Slattern returns, blocking the Jaegar's path. With no way around Slattern, Gipsy leaps foward and tackles the massive Kaiju, sending them both falling into the abyss. On the way down, Slattern uses its tails to stab Gipsy's back, which damages its air supply. But before Slattern can cause any further harm, Gipsy's pilot Raleigh fires the nuclear turbine that blows a whole straight through Slattern's chest, finally killing it. Gipsy Danger is then able to use Slattern's corpse to pass through the breach successfully and detonate its nuclear payload, destroying the portal and also killing the Precursors. 

Trivia

• Guillermo Del Toro has cited Chernabog from Fantasia as an inspiration for Slattern's appearance scene.
• Slattern is the largest, strongest, and the presumably the most cunning kaiju in Pacific Rim.