‽- Something your muse has problem in understanding
Hahaha, this is loaded question as it involves a large part of her character. She quite honestly has a hard time understanding emotions, human life, and existence. She’s a soul that has had to live most of her life in the dark and cold, devoid of life and killing any life that came into contact with her, her existence never to be known. This caused her from a very early age to numb any emotions she could ever really feel as it only would make her carrying out her duty more difficult. She had to be content with essentially not existing in the minds of others. Pluto had her moments when the Silver Millennium was thriving and she treasured those dearly, as they were the only times she felt she could not be seen as just a myth.
When granted with the opportunity to live amongst others and carry out a somewhat “normal” life, she has (and is) trying very hard to be rid of the numbness she was so accustomed to blanketing her. As Setsuna, she wants to be able to feel every emotion possible to make up for what she couldn’t, wouldn’t feel for so long. In a way, it’s her trying to reach out for a humanity that she never really had the pleasure of knowing or having herself. Naturally, it’s difficult for her to understand at times—she doesn’t always know what to do with certain emotions and she doesn’t always know what the best ‘human’ way to react is, and this can make her come across as distant. She’s getting a better understanding of it while being with her fellow soldiers and sisters-in-arms, but she still has her days where she has to sit back and try to process everything around her so she can actually understand what she’s feeling, what her existence means, and what being human is really like.