Slender the most known of the siblings and probably the most powerful if we look at power as in capabilities and not money (splendor is the richest afterall)
While offender can maul you to death, Splendor can tear you limp from limp, trender can impale you with his tentacles/ appendages (idk how to refer to em) and tender hypnotize you to take you off your guard to then kill you to eat later, slender can do so much worse.
While not only being able to psychologically torture you and force the slender sickness (prob gonna take about that in the future) opon you in a strength than can kill you pretty quick he also has strength you wouldnt expect from such a boney stature.
Though he prefers to let his proxis kill so he can focus on stalking and his paper work. Yes even a powerful killing monster has paperwork. Like his siblings he owns businesses. Most often book or archiving related. Probably some law firms too just to hold another ounce of power and hopelessness to victims.
He also is quite popular for his kind because of his success. Yet he doesn't go to a lot of parties if they aren't business related. Preferring party's of people he's close to like his siblings.
He is demisexual and pansexual. Thoug he is indeed hard to even get a close bond with. The closest someone has managed to get to him other than his brothers and parents are Sally and his 3 proxis Toby masky and hoodie. But those are more like a parent and child kinda relationship in his eyes.
Even if they have that relationship though slendy is not a dream dad or a very good one. He has his knick knacks that don't only include him being a cryptid.
Of course the psychological torture hoodie masky and Toby had to go through even if he did save Toby in a way, they were still very bad and no one likes to think if those things as a happy memory. Also he is seen as a boss for the proxis too so. A powerful at that that could hurt them anytime he pleases so they are wary of him. Not to be panicking 24/7 but always a thought in the back of their head and slendy is not one to show emotions open a lot.
For Sally it's the fact that yes she sees him as a parental figure but he is very much imperfect. Not showing a lot of emotions and making her feel a disconnect from him but also because of his workaholic nature
He may be cold, a workaholic and not really a good guy, he does have some good personality traits. He's a good listener, has a observing nature so he knows when one of them is feeling depresses or anxious and he knows that he shouldn't overwork his proxis. He's calm and doesn't like screaming unless someone tries to threaten his siblings or proxis.
His hobbies include violin, playing the piano, reading, watching the history Channel to check for wrong information (he's very old he knows some events very well) do news paper things (whatever you call those games in there. Forgot their English name) and likes to garden.
He does have other creeps living in his manor but he doesn't refer to them as real proxis and just uses the word proxi for them to not have to use extra words that others might find confusing.
He didn't aquire them like his proxis but more as he met them one day per chance or because he was interested in them and their story and we'll most don't have a place to stay or live so they might as well stay with him and do some work to be allowed to eat and live without constant fear of what to do next to survive.
Jeff and ben totally were the first after his proxis to join the manor. Then eyeless jack and laughing jack before some others slowly joined. (Not all the creeps. Some prefer splendor or have their own stay)
The rake can be seen as an inhabitant of the manor but he prefers to stay in the forest and just show up for social interactions, food (the sweet stuff) or to just sleep somewhere warm or dry for a night or day. Slender has a smaller room extra just for him to have his alone time.