I had a Zoldyck DR where I was Illumi's twin, so this will be based on the anime and my experiences.
Illumi is a rather complex character, out of everyone in hunter x hunter, I think he's the most misunderstood in my opinion. Most people's understanding of him is that he's emotionless, every bit a product of the environment, which to some extent is true, however a large part I think is his own nature.
The only part of him that's emotionless is his face sometimes, in simpler words he has a bit of a resting bitch face. If you really analyse the scenes he's in, he does a lot of things just for amusement rather than following every word of protocol like everyone thinks he does. Sure he might be a perfect assassin like the Zoldycks raised him to be, but their standard for assassin isn't just kill without thinking. It's to put the job above everything else and perfect your missions. This is also part of why Silva let Killua go, because he thought Killua would realise himself that what's most important to him and what he's best at doing is killing, they didn't want him to be mindlessly killing under Illumi's needle forever.
So Illumi having his own side quests or interests does not affect the fact that he's a perfect assassin. Like when he let that guy go without killing him because he wanted to die fighting with Hisoka. He really had no gain from letting him go, he simply thought why not. If it doesn't go against his goal he has no qualms against it. Except when it comes to Killua, I'd say Illumi is a reasonable person. Another example is when Gon asked where they live and he simply told him, because he does not believe it goes against his goal, he didn't believe it'd hinder anything and Gon would inevitably find out.
Another example is that he helped Hisoka with Chrollo simply because he knew him longer as he stated. Aside from his goals, he really goes along with the flow because why not. Something I think of when I think of his personality is nonchalant dreadhead HAHA. He's more nonchalant then emotionless. He has his own wants and goals, like when he wants to control Nanika because of her powers. I think his cackling was a little scary, however it's a display of emotion. Or when Hisoka purposely made him mad for him to release bloodlust, and he asked Hisoka if that was on purpose with an annoyed face. He has emotion just like anyone else, he's just very matter of fact when it comes to his goals and other than that he doesn't care too much.
An example from my DR is when a couple asked Illumi to take a picture of them really quick on the street, he said sure and willing did it and left them be.
Of course due to assassin training since he was young wasn't good, but I wouldn't say the way he acts is fully from trauma. He was also sheltered when he was younger, there was no temptation of things he wanted which is different from Killua. Then when he was involved outside his family he never got closer to anything or anyone enough to prioritise over his family, and I don't think anything will. I think due to his upbringing he just doesn't find joy in many things that would be temptation to others, and he was never loved in the way conventional love happens, to him, love is your blood ties and importance.
It's not that he's broken and someone's healthy love would change him, his mindset has been raised that way, so healthy love wouldn't speak to him nor would he enjoy it. It's just not how he feels affection, like for example your love language has always been gift giving due to your upbringing, you feel joy when receiving gifts because you've associated that with love, so when someone shows you physical affection that's not what you associate with love so you don't feel joy. He's an extreme example of that due to shelteredness. It's not trauma it's just the way his upbringing cemented in mindset.
Another reason people associate him with emotionless is due to him not understanding simple things, like with his conversation with Hisoka, he thought he's one of the people Killua loves most. This isn't him being crazy or broken, it's just his mindset, I think he's a really good example of nurture and nature combined. To him family and killing is most important, and so Killua being extremely talented and raised like him, he believes deep down it's the same for him. He kind of projects his own wants and beliefs onto Killua. Raising Killua to be the perfect assassin is a goal of his, it's etched so deep in him because he's always been in control of everything. He controls needlemen, he's a manipulator by nature. He believes Killua is apart of him, and that Killua's future ties with his, which is why he was also happy to die for it, like in the chairman arc. It wasn't a deceptive lie. He loves Killua in his own twisted ways because he really believes Killua is apart of him, like other Zoldyck members are also his family and important but his love towards them is different because they are his family but Killua is closer. His project, his creation, his nurtured subject.
It's like an owner would think their dog's behavior is a reflection of their training, if they misbehave they weren't trained right, not that behaving a certain way was never in the dog's nature. Illumi is a little slow with a lot of common sense, because the common sense he was raised around wasn't the norm. Milluki is very chronically online, Killua was curious by nature and remembered things as he learned, but Illumi didn't bother to analyse or understand the world socially because it wasn't apart of his goal, in one ear and out the other. Like how many elderly don't care for the internet because they weren't raised around it and don't care to learn.
Anyways, overall I think he's a very goal oriented, aloof and calm person. He minds his business, focuses on his goal and everything else he could care less for, unlike the way he's painted as emotionless, he very much can enjoy things, but only when his goals are not involved. I believe if he was given the time he would pick up a random hobby like crocheting.
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My Shifting Journal | Yona yuuki
FanficHello! This includes everything you need to know about shifting, including ideas, inspirations and more! Comments are also open for any question!! Some realities I been to are: My Hero Academia Hogwarts Holiday DR Miraculous Ladybug