Park lost her partner after going Kaiju mid Jaegar fight.
(http://25.media.tumblr.com/9607853e3d0bcc6b953fbf462b488508/tumblr_mqaqdz3z1t1r1eny6o3_1280.png)
With their psychic bond gained during the events that lead to Impact Day, Arena and P2 were a natural fit as partners for the first line of Jaegers built.
Their Jaeger's hemispheres complement the pilots' powers; Arena's arm can morph into a variety of additional weapons, and P2's arm can be charged with graviton particles that "hit like a black hole" as P2 puts it. While it's not sure what this means exactly as the punch does not create black holes, if you were to ask a kaiju, they would say that regardless, it hurts.
(http://25.media.tumblr.com/1b27d2c347c30c6759940a7d2b6269cc/tumblr_mqaqdz3z1t1r1eny6o2_1280.png)
When the first kaiju wave made landfall, all the Vektrans in Void City did the logical thing, and packed up and left. All except Miller, who joined the fight as a Jaeger pilot.
Being a well-collected psychic, Miller takes nothing with him through the Drift with a partner, allowing him to sync up relatively well with any other pilot. But general distrust of Vektrans has left Miller with no partner that has stayed for more than a few missions. With no one willing to pilot a Jaeger with him, he has been assigned as the dreaded substitute for any Jaeger who is lacking both pilots.
(http://24.media.tumblr.com/e489fadc14edbfd535bb180a9c637c7a/tumblr_mqaqdz3z1t1r1eny6o1_1280.png)
Park Kim performed relatively well in the tests, but once she entered her first kaiju battle, the city learned a horrific truth; Park herself could turn into a kaiju, and an uncontrolled transformation as she burst out of her own Jaeger cost her pilot his life (though the kaiju they were battling was destroyed by Park, whose kaiju form has been classified Category IV, Gigara). Park has been under strict surveillance ever since, as the city decides how best to weaponize her.
Defying everything science has known about Drifting, the immense hatred of each other shared by Elle and Edge has made their Drift compatibility incredibly strong yet also become a PR nightmare for recruiters. The only thing they hate more than each other are Kaiju.
(http://i.imgur.com/vuhbUHs.jpg)
Their Jaeger: the Buster Bullet, armed with twin chest-mounted Particle Projectile Cannons as their special weapons and Stabbing Fingers for melee. Carries a nuclear power plant on its back due to the insane power requirements of the cannons.
(http://i.imgur.com/PmvjS2x.jpg)
Unsubstantiated rumors among the support crew suggest that the pilot's hate is each other has intensified due to someone referring to the Jaeger as having "PPC tits."
[7/24/13 10:37:55 PM] Ember: Miller and Wiz are Jaeger pilots now.
[7/24/13 10:37:58 PM] Ember: It has been decided.
[7/24/13 10:39:34 PM] Ember: *rolls away*