Doctor Ciel (シエル Shieru) is a supporting protagonist and the main heroine of the Mega Man Zero series. A human scientist, Ciel sought out the legendary Reploid hero, Zero, to fight back against the tyranny of Neo Arcadia. Ciel leads the Resistance to support Zero. Ciel is a young girl with a modest stature. She has pale skin, cobalt blue eyes, long thick blonde hair with three angular bangs, usually tied into a ponytail. She wears a pink outfit consisting of a helmet, a sleeveless dress, a resistance vest, plated high heels and white gloves, with a black bodysuit underneath. The helmet has a hole on the back for the ponytail, and a retractable white visor. She also has winter clothing composed of a long pink jacket and thick boots. Ciel is a gentle, kindhearted person who is always concerned about the welfare of others. She has an issue with insecurities as a result of Neo Arcadia's pressure on the Resistance, but Zero's continued support gives her the motivation to move forward. She cares greatly about Zero and often regrets requesting his help for dangerous missions. She is also close to Alouette, having named the girl and given her the stuffed animal she is always carrying.

Despite her age, Ciel is a genius scientist, already capable of creating a replica of X merely by observing the methods used to create the Four Guardians. Her study of Cyber-elves also allows her to develop a new energy source in the Ciel System, an important discovery that starts a new age in the field of energy research. Ciel believes in solving problems not through fighting, but through the power of science; she can be very dedicated to her research, to the point of staying up late. Ciel also has an emphatic connection with Cyber-elves, and illustrations often depict her alongside them in order to reflect this.

Throughout the series, Ciel is shown to have some romantic feelings for Zero, regardless of the fact that she's a human and he's a Reploid.