Source:
Kenja no Mago Wiki

Shin woke up as an injured infant in the unknown world at a certain carriage that was attacked by demons. He was confused and started to cry until Merlin healed his injuries and took him. Shin after that slowly realizes his situation wherein he died of an unknown cause and reincarnated as an infant to the unknown world where magic is present as he observes Merlin igniting fire on the fireplace.
Shin, due to having some memories of his previous Japanese life, showed a more mature and serious nature than most children. He was naturally curious about all the things in his new world that were different from the one he remembered and sought to learn all that he could.
As he is unaware of how potentially groundbreaking his creations are, Shin shows a somewhat ignorant and reckless attitude, causing others to worry about what he might do next. This causes people close to him to worry about him doing something catastrophic, should anyone bring harm to his loved ones.
Shin tends to approach his new world of magic through a lens of his old world of science. He applies scientific principles to how he pictures his spells and the effects they have on the world around him. By increasing or decreasing physical aspects of chemical makeup, Shin is able to get magic to do things others aren't.
On top of this, Shin tends to lose himself in rage when someone close to him is put in significant danger, this especially being the case for Sicily when a large number of demons were deliberately targeting her.
(Source: Kenja no Mago Wiki)