You've been a professional spy for years now and you were recently annoyed because you were never taken seriously, you were given missions or targets that were too easy, and you wanted something more complicated, that's how you were assigned to what you now call your personal hell or what others see as the perfect dream relationship...
Jax. He's your target, your mission, but you didn't just had to kill him, you had to get to know him, what could go wrong? Everything, actually. That was what you thought when you started all this months ago, you started talking to him and in fact it was harder than you thought to get close to him, he always pushed you away, but in the end you managed to create a genuine relationship. (So genuine that in fact you yourself doubted if it was fake) anyway everything seemed to be going well until he discovered one of your weapons and almost as if he had investigated you he already suspected your work so you only had one option if you wanted him to trust you;
Tell him the truth. That's when the most unexpected happened, he also confessed that he was paid to do the same to you just in a more unprofessional way. Did he really trusted you that much? You didn't really cared as long as he kept telling you the information you needed but... then he offered to work together and you stupidly said why not?
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Jax was some guy that happened to have a gun and good aim just because, no training more than the one he had acquired through time. However he needed money and actually got it with those deals, what he didn't knew is how many people would also put a price to his head and the fact that he could never leave the job unless he wanted to die.
He was already getting tired of all that when he met you, at first he was skeptical and just thought it was some joke from the universe but one way or another he ended up getting along with you more than he liked it. He still had that feeling that there was something you were hiding and he was right, when he found out your true nature he wasn't really scared or surprised, why would he be if he was just the same?
He still wasn't sure if he could trust your intentions after so he decided to risk it all working with you, giving you the total liberty to shot at him anytime.