When talking about Sith!Luke in the past, I always used to base my headcanons on RotJ Luke, since his slightly more withdrawn and mysterious demeanor, as opposed to his character in the very beginning of A New Hope for example, better fit the (concerning and unhealthy) narratives I wanted to create.
But THEN, recently, it hit me. And it's so simple, really. I don't need a serious persona for that!
• imagine Sith!Luke...but vibrant
• brimming with a power stifling like the heat in summer and blinding like a sun
• not a stoic and cold young man but someone dangerously excitable and passionate
• mostly about morally questionable things
• (the wrong things some might say)
• but spirited and light-hearted nevertheless
• a little bit whiny, a little bit impatient (very impatient)
• eager for new experiences and the unknown
• an overwhelming presence in all aspects
• loved by his family and deeply feared by everyone else
• possibly the most spoiled person to ever exist across entire galaxies
• and then, with growing age, add to that a terrifying amount of entitlement and potency
• he never hesitated to loudly voice his desires before, to demand to be seen and/or heard
• now that he is older he doesn't ask for anything anymore
• his privileged and meticulous upbringing has taught him to command those around him and act on his wishes
• no longer does he go to such lengths as to wait for his father's approval on his actions
• entire solar systems are his playground throughout his youth and young adulthood
• nations sit by and watch as it slowly becomes apparent over the years that even if he occasionally interrupts or crosses his father's plans...
• ...nothing substantial happens
• Vader doesn't reign him in, doesn't enforce consequences or retaliates
• even under the Empire's reign the speculations and gossip can't be depleted completely
• and what started as a fearful whisper here, and a maliciously gleeful claim there, slowly but surely evolves into a widespread belief
• the Emperor's son, Luke Skywalker, has surpassed his father in both power and strength
• Vader is as helpless against him as is everyone else
• it makes sense, explain the self-proclaimed experts, that even he knows it to be unwise to have his not-entirely-human, eldritch horror children turn against him
• he would not have a chance against their joined wrath, they insist
• (alternatively Vader just really loves his son and there IS a heart beating somewhere beneath that black cloak of his)
• of course nobody is ever truly able to tell whether any of this is actually true
• the Skywalker family always presents as a unified front, unbreachable and unwavering in every aspect
• what freaks people out the most about Luke though, is the stark difference between him and...basically everyone else in his immediate environment
• something about him, especially in direct comparison to Vader or his sister, is immensely unsettling
• (more so than they already are to begin with)
• not only to his enemies but to the Imperial staff as well
• imagine you're just a simple guy, trying to do your job
• maybe you're new, a recent hire
• and suddenly you run into either of the Skywalker family
• you'd be stressed, of course, because who wouldn't be
• but there is some comfort in predictability
• and Vader, for example, is predictably cruel in a way everyone is uncomfortably familiar with
• he enters a room shrouded in black like his father, death incarnate
• (of course this version of Luke is impeccably dressed as well)
• (he's featured in most galaxies equivalent of the Vogue. His role as the heir to the Imperial throne is merely an unimportant side factor to those who appreciate and celebrate the heights of fashion)
• and then he allows his hood to fall away to reveal his face
• but somehow it's a little bit too bright
• sometimes it makes his eyes gleam in a way that tends to be subtly offsetting
• no one is able to really interpret its quickly shifting meanings accurately
• is he laughing about a joke or about to crush you like a cheap soda can?
• are you going to end up as his personal jester, an entertainment source for the foreseeable future, because he thinks you amuseable?
• is he planning to feed you to the new pet he recently acquired?
• does he want to bed you?
• certainly not you or anyone else
• probably not even himself
• Luke goes with the flow
• he's a man who appreciates good opportunities
• please him a little too much and you may find yourself in situations you'd rather avoid
• the same can be said for the opposite
• his smile, however, will not waver
• it can convey charm, amusement, delight, wonder,...
• but tips into madness as easily and quickly as he is able to murder you in cold blood
• more often than not it makes him look slightly insane
• often during battles and in anger it's like his face disappears and in its place steps a grotesquely contorted mask
• it haunts those who survived him
