Deadly assassin few get the privilege of seeing, let alone talking to. Based on the film 'Crouching tiger, hidden dragon' - Khajiit with tiger aesthetic that uses his dragonblood to master shouts involved in stealth (aura whisper, throw voice etc). In large areas he chooses a spot and takes out all available targets with poise and precision, if any remain he moves to another location to finish the job. In smaller, narrower tunnels and caves he will rely on his dagger.
Can be considered a split personality: ordinarily he walks about in plain clothes helping anyone who asks and is a perfect gentleman to whomever he meets, speaking in his soothing and polite khajit voice. He builds a reputation as a man who can make things happen but no-one knows how he achieves this; it is asked of him, and a few days/weeks later he returns.
In the field, he wears the ancient shrouded get-up but with the nightingale body armour. He uses the blade of woe and nightingale bow primarily but has zephyr as a backup bow (plus one other: see later). As a nightingale, he can use Nocturnal's gift of brief invisibility and so in combination with the shadow stone this can grant him a total of 3 minutes invisibility to get him out of tight situations.
Factions-wise, he obviously joins the dark brotherhood and thieves guild where his strengths are best applied. However, even these circles have no idea who he truly is; they heard of his reputation for mysteriously making things happen and recruited him.
*There are some enemies that are simply too powerful to sneak up on or take out with a few ranged sneak shots and for these he uses the ebony blade. These include dragons, dragon priests etc. Due to his personality he has no trouble finding friends he can betray to return the blade to its full power but he chooses the targets wisely so he doesn't affect his future too majorly.