Originally, Miyu has a mature, dutiful, and rather unfriendly personality. She takes her duties as a Magical Girl very seriously and is initially dismissive of Illya's abilities. Despite being the age of an elementary schooler, her knowledge in physics and mathematics is at a university level; because of her scholarly mind, her communication skills lag substantially behind her age group as she tends to view everything more logically while explaining or exploiting the hard facts behind it, not understanding the emotions surrounding them. This side of her expresses how she lacks imagination and hints a sense of loss in child innocence. Despite this, she is shown to have vulnerable sides such being embarrass and being timid towards those who conveys acts of deep kindness and other possible overwhelming emotions like obsession.
When she opens up to those she becomes close to (mainly Illya), she exhibits the capacity to deeply care and is very dedicated towards them. This creates two sides to her personality where she can very affectionate and devoted to someone she cares for, but extremely cold and dismissive towards anyone else. Miyu seemly doesn't know the difference between friendship and love, switching instantly from aloof and distant to overly clingy, but Illya accepts her anyway and returns her friendship. Notably, Miyu sees Chloe as a rival for Illya's affection.
It is revealed in the past that she was originally stoic if not emotionless due to her duties of being the Holy Grail but showed sense of attachment. Her interactions with Shirou, who she eventually calls her brother, soon brought more color and emotions to her world while also bringing deep guilt and burden in her heart to the point that she was shocked and conflicted when she finds out that Illya's adoptive older brother was another Shirou Emiya. This, along with the Ainsworth's treatment towards her, unveils the reason behind the personality she had when she was first introduced. Because of her upbringings, the guilt, burden, and turmoil within herself brought her great despair that she almost gave up on the idea of hope, but after Illya and her allies/friends saved her, she begins to show resolve. Like Illya, Miyu also seeks a path where everyone can be saved.
She comes to value Illya's friendship greatly and is highly protective of her. This dedications goes as far as willing to risk her life for the sake of their friendship. She shows another side of her personality when she is around her brother, where she behaves like the young girl she actually is. These behaviors include wanting to be spoiled, like to wear animal onsies, and throwing a childish temper.
