Apex is the alias of the ex-Apex Pilot known as Connor Pemberton, previously known throughout the Wasteland as the Sole Survivor.
Connor came up with the idea of Apex 5 years after the events of Fallout: Pathfinder, after his body suffered a major age shift which made his physical form progress by approximately 40 years in seconds. His form became old and ragged, and he quickly started to lose strength and agility. To avoid being a burden on those he cared for, Connor vanished into the Glowing Sea and left everyone a message saying he was never going to return for their own safety, and as such rumours of his death began spreading throughout the Wasteland like wildfire.
2 more years later, known evildoers and criminals started being picked off by an unknown assailant who left little more than a calling card with the word "APEX" scrawled across it. As the years passed, hopeful Settlers began to hope this so-called "Apex" would be there to protect them in their time of need.
After 3 more years, Apex became a world-infamous vigilante with a tendency to kill any he desires. After Kate had a close encounter with a Slaver gang, Apex saved her and consequently LittlePip only to thrust his known self-aiming pistol into the mare's neck and consider killing her. However he thought against it and despite Kate asking him to, he refused to travel with her. Instead he handed her three of his famous cards and told her to burn them if she needed him. Two of these three "Burn Cards" were used by Kate in combat and thus Apex appeared and slew her targets, while the final one was used inside the Omega Platform minutes before Kate entered the Psionic/Magical Siphon Override Pod, so he could witness her final moments. He told her that if he could, he would've taken her place, and after she entered the pod and the Omega Platform was prevented from exploding he left another of his Burn Cards on her pod, except this time it was face-down to show it was out of respect and honour for her sacrifice. After that time, he wasn't seen again. Even his Titan was unable to pinpoint his location, as Connor had severed their Neural Link much to the distress of both of them.
Though it is unexplained, Apex may have some Psionic abilities, unseen contacts, or has simply been observing Kate for a while before their official meeting as he both knows her name immediately and knows about her previous endeavours. For example he not only knows she entered the Divinium Confectionary Factory with Amanda, Locke, ED-E and LittlePip but also knows that she consumed Experiment 1971 and knew not to touch the raw, unrefined Divinium stored inside the factory's bowels. He also congratulates her in managing to kill the Alicorn calling herself "War" by dropping a Dynamo Bomber onto her head while simultaneously scorning LittlePip for doing the same, albeit with a train car and not a bomber. Due to his past experiences with her, Apex hates LittlePip to the core.
Apex is described to always wear a full set of HoloPilot gear with a glowing red visor and decals, with the emblem for Apex painted onto the helmet's faceplate and the torso's main chest-plate's left half. His armour is dark, making him harder to see at night, and is apparently always worn by it's user. He is armed with a self-aiming revolver pistol known as the B5 Wingmare, a gift from Scootaloo after his last encounter with her. His actual organic body is very old and scarred due to many conflicts nearly ending with his demise, and his actual face is only seen twice. Once by Kate upon their initial encounter (where the left half of his helmet's faceplate is blown off thus making his face partially visible), and again in the Omega Platform where he removes his helmet entirely while trying and failing to convince Kate to abandon her idea and flee to CHS, where she would be safe and survive.
Apex is said to have access to the Pilot abilities Cloak, HoloPilot, Stim, Particle Wall, Grapple and Pulse Blade. Much like during the Equestrian Civil War, Apex will often use multiple Pilot Abilities at once to intensify them and make fighting easier. For example he will deploy a HoloPilot and Cloak at the same time to make his true whereabouts very difficult to pin down. Another example is using his Pulse Blade and shooting it with his Grapple Hook to make a portable detection system.
How Apex knows when someone is using one of his Burn Cards, how he knows where this is happening and how he gets to the area so quickly with minimal detection is unknown, though Amanda theorises that he is utilising a form of "Phase" to travel across the Wasteland in seconds. However prior to the Last Lights, Connor is never seen using Phase and claims that time travel gives him a headache, thus he would never use it. Apex himself never explains how he moves almost instantly across the world, and how exactly he knows when a Burn Card is in use. He says he "just knows."
Upon becoming Apex and aging quickly in a short timespan, Connor has become reliant on Stims to keep his body active and able, giving him seemingly supernatural movements and reflexes, able to interpret an attack before it is cast. This also gives his melee strikes far greater strength than he had before, allowing him to completely shatter a person's skull in a single rapid punch seemingly undetectable by the naked eye, and he can punch through two inches of Depleted Elerium plating. This is a condition listed as an addiction by Kate. He claims he could quit using Stims at any time, but does not do so as he knows if he did his body would be too weak to move at all, let alone fight as an agile Pilot. His normal movements are maintained by a steady flow of weak Stims, while as he engages in combat his dose is increased exponentially. As a Pilot, he has a limitless amount of recharging Stims. When he does enter combat, his strikes are so quick that they are nearly undetectable by the naked eye, and even lock-on systems have a hard time tracing his movements.
While no longer the wisecracking "typically British" soldier he once was, Apex still retains similarities to his former self. This includes some of his Executions such as Unarmed Combat, 2-3-2 Combo, Curb Check, Seeing Double and One-Inch Punch. He's also seen performing both an Execution used infamously by Rainbow Dash before the Last Lights (known simply as "Mach 5") and a brand new Execution that Kate calls "Hailstorm Jab." This reflects his brutality, which he also retains from his past self. However he no longer has any sexual drives or concept of non-familial love. He retains some of his previous nature as a sarcastic individual as when Kate questions how he knows what Vault she is from, he points out he looked at the big "115" on the back of her Jumpsuit. He also retains his fear of the ocean. When Kate asks if he was one of the heroes like those she had heard of during her travels, he paused before turning back to her and saying "Not anymore" before Cloaking and vanishing from her sight entirely.
Apex is always hesitant to talk about his past, especially his daughter whom he feels he abandoned even after he went through Hell to find her.
While he was once able to a Pilot Core Ability "Flame Core," he has since lost this skill and it has instead been replaced with "Overdrive" which increases his Strength and Agility to what Kate discovers is a whopping 50, while most other people's SPECIAL scores cap out at 10. However he also claims that use of this Pilot Core Ability is exceedingly dangerous, and leaves him depleted after it expires.
It is hinted that Apex has had an experience with William Vivaro during his 2 years missing in the Wasteland, as Apex notes Kate is looking for a face that he knows and William explains how Apex once saved his life from New Lunar Order Headhunters. He also has one of Apex's Burn Cards on his robotic body, tucked into the side of the brain containment unit.
Apex's first actual appearance is far from his first mention, as one of the first things Kate witnesses in Bolt City is a well-known criminal hanging from the city's brothel with one of Apex's Burn Cards in his jacket, his face beaten in and his spine protruding from his back as city officials try to clean up the remains. There are then multiple occasions where Kate sees a deceased individual and a Burn Card on their body. As Kate is randomly assisted by the Mysterious Stranger and she questions Apex about him later on, Apex claims he had nothing to do with her past experiences and states the Mysterious Stranger is a lone gunman (like Apex) who helps the good defeat evil, but the two are not connected. Apex has also been visited by the Stranger many times both prior to his transformation and afterwards.