sanji backstory spoilers

Sanji was born under the name Vinsmoke Sanji, which matters because the Vinsmokes are the royal family of the kingdom of Germa and are home to the legendary military faction Germa 66. Sanji's father, Vinsmoke Judge, experimented on Sanji and his siblings in his mother's womb, attempting to make them genetically-modified super-soldiers without a shred of empathy or emotion to lead his perfect army. While his brothers Yonji and Niji and his sister Reiju were thus modified, his mother found a way to prevent the process from affecting Sanji, leaving him able to feel emotions and form relationships but also leaving him completely normal. In Judge's rigorous training, Sanji consistently fell behind in every single metric, left completely in the dust by his siblings. Between Sanji's physical normalcy and empathy for the servants of the Vinsmoke house, Judge rapidly began to view Sanji as a complete failure- not just as a soldier, but as a son and as a human being.

Sanji's life only got worse from there- he had bonded with his dying mother over time (the other children couldn't care less, as they were wired to feel nothing for her), and had picked up on his mother's love of cooking as a result. Regularly punished for cooking food for the mice and vermin in the house, Sanji was eventually left to rot in a prison cell by Judge, who declared to the Germa kingdom that Sanji had tragically died. Stuck in a cell with an iron helmet covering his face, Sanji thought he would die alone and ignored by the world, but his sister Reiju helped him escape on the condition that he never come back and never call himself a Vinsmoke. More than happy to oblige, Sanji left to sea and eventually found his way onto a ship, where he got work in the ship's kitchen, leading into the rest of the bio.