Trespasser is the first Kaiju to emerge from the Breach and attack humanity. It attacked San Francisco on August 10, 2013, and over the course of six days, it destroyed three cities in a thirty-five-mile-long path of destruction, before it was finally killed by the United States Military and British Royal Air Force.
Little factual information could be ascertained from Trespasser following its attack in San Francisco. It stands taller or at level with the Golden Gate Bridge, and is strong enough to tear the structure apart with its claws. Trespasser's skin was thick and protective, and could withstand almost any type of conventional weaponry. Trespasser emits an orange, bioluminous glow from inside its mouth. Like Slattern, Knifehead and Scunner, Trespasser's main arms are actually two fused arms, each ending with three-clawed digits.
Trespasser's approach to the city is marked with a 7.1 earthquake, followed shortly by its rise from the water near the Golden Gate Bridge. It destroys the bridge and everything on it, proceeding to cut a massive path of destruction through San Francisco, Sacramento, Oakland and the numerous surrounding cities. Conventional weapons used by the US military, assisted by the Royal Air Force, did nothing to penetrate the hide of the creature.
The fight against Trespasser lasts for six days before the military would finally resort to using three tactical nuclear missiles to kill it. The three missiles killed Trespasser, but at the expense of Oakland, much of the Bay Area, and all of those living there.
Shortly after its death, the military gathers its remains and put its skull on display for the public. Trespasser's attack is thought to be an anomaly by the general public; however, six months later, a second Kaiju emerges in Manila.
Because Trespasser’s attack occurs before the formation of the Pan Pacific Defense Corps, when established, the Defense Corps is unable to determine its category class from its corpse after the creation of the Serizawa Scale.[note 1]
- Scunner and Knifehead use the same basic CG body as Trespasser in Pacific Rim.
- According to Travis Beacham, "Axehead" (commonly spelled Axe-Head) was a nickname, referencing the look of the Kaiju, before it was given the name "Trespasser".[note 2]
- NECA's Pacific Rim Series 3 line included a Trespasser figure. In October 2014, they released another Trespasser figure for the Deluxe Series named "Axehead".
- "Axehead" is also the name for Trespasser in Pacific Rim: The Video Game. It appears much bulkier than in Pacific Rim, and can run on all fours.
- Trespasser also appears in the Pacific Rim: The Mobile Game and Pacific Rim: Breach Wars.
- Trespasser was also called as San Francisco during the production. 
- Trespasser's introduction is a likely reference to the Spectreman episode, "The Wrath of Moonhunder!!". Spectreman was one of several Tokusatu series Guillermo del Toro watched as a child.[note 3]