His name and character design is based on Justin Goncalves, one of the game developers (devs).
Justin is a powerful clone soldier appearing in the final mission Devs De La Muerte. Justin and Mike fight against player's squad in the boss battle. Although Justin is Ninja class, that helps him only in traits he has. He is easier to kill than Mike due to his lower health.
He uses custom sniper rifle Rehab, which is the custom version of Intervention. His secondary weapon is Jericho pistol. His Rehab combined with Wall Hack killstreak which he activates middle in the mission makes him more deadly than Mike, as Mike's Defriender shotgun can reach the same range as Rehab, but is inaccurate. He uses Engineer's Shotgun Turret killstreak. Also, sniper rifles normally can't be used by Ninjas.
Along with Mike, he also uses many unlimited killstreaks of different classes, which he changes as player scores more kills. They are (in order):
After "I'm The Juggernaut" he also has Chameleon trait, which is normally used by his class.
"I have an idea! Stick to the shadows!" - Justin to Mike when they turn on the Chameleon transparency
"This isn't working, bring on the hax!" - Activates Wall Hack.
"That's it, DOUBLE WEAPONS!" - Getting angry and activating the final killstreak.
Justin is easier to kill than Mike, due to his low health, but on long ranges, he almost never misses with his Rehab which is able to kill you from one hit. It is highly recommended that you kill him first as his Rehab is more deadly than Mike's Defriender, especially when Wall Hack is activated. Use Life Support and Adrenaline skill.
Use soldiers at the max level for Hard and Insane modes. Use custom weapons if possibile. Try to pick them off from far away using Rocket Launchers or a Sniper. One shot kills are the best, as they won't have a chance to retaliate. When using an Experimental, DSR1 is the best choice as they have lobbed range and extremely fast grenade speed within long ranges. Iron Curtain rifle is also useful as its very high fire rate allows you to rip through enemies from any range. Equip Nanomachines to heal faster.