Sado is a teenager of Mexican descent. He is dark-skinned with pronounced cheek-bones and has wavy brown hair with bangs that generally hangs over his brown eyes. He is a muscular and extremely tall man and appears to be much older than he actually is. Sado has a tattoo on his left shoulder which reads Amore e Morte ("love and death" in Italian) and consists of a heart with a snake and angel wings around it. He typically wears either his school uniform or flashy short-sleeved shirts with open collars. He also wears a dark, long-sleeved shirt with a rose emblazoned on the left sleeve. Seventeen months after Aizen's defeat, Sado has grown a small goatee on his chin.
Sado is a quiet person and is sometimes the subject of jokes or bullies because he never fights back. Nonetheless, he is treated equally within his group of friends in Karakura. Sado also has a soft spot for cute things, like small animals and Kon's plushie form. Sado is very intelligent: he ranked 11th out of 322 students in Karakura High School. Sado has a very loyal personality. He is quite loyal to his friends, especially Ichigo, who is one of his closest friends, even going as far as to try and defeat a Gotei 13 captain to go help him. He is also fearless and told so by numerous enemies such as the first Hollow he fights. He displays little emotion except when something he cares about is in danger.
Sado's favorite food is tomatoes. He also likes music, but rarely watches TV. He only likes two shows, one of which is Carnivore Kingdom, which starts at 8 a.m. on Sundays. He likes it so much that he bought the DVDs for it. As Kon is a cheap knock-off plushie of the main character of Carnivore Kingdom, Sado finds Kon especially cute. The other show he likes is Don Kanonji's Ghost Bust. Sado also has a bass guitar.
Sado was born in Okinawa, Japan, but was taken to Mexico by his parents at a young age. After their deaths, Sado was adopted at the age of eight by Oscar Joaquín de la Rosa, a man believed to be his grandfather since Sado calls him Abuelo ("Grandfather" in Spanish).
During his early years, Sado was extremely violent; exploiting his advantage in size, he intimidated and hit any other children who annoyed him. Oscar Joaquín tried to teach Sado to be gentle, but was initially unsuccessful. One day, the fathers of the children Sado fought, seeking to punish him, attacked Sado. Oscar Joaquín stepped in between them, taking Sado's punishment without retaliation. Sado was inspired by this example to become a decent person. To commemorate this event, Oscar Joaquín gave Sado an old Mexican coin; years later, when Oscar Joaquín had died, Sado made a vow that he would never fight unless it was to protect others, as instructed — even if his own life was in danger. The coin would come to symbolize this vow, which Sado now claims to hold more value than his own life.
Some time after De la Rosa died, Chad moved back to Japan and entered Mashiba Junior High School at Karakura Town. In the 8th grade, he helped Ichigo when the latter was attacked by thugs. Chad was later attacked by two of Yokochini's gang members and, keeping with his vow, did not strike back or defend himself. Ichigo intervened and saved him and later learned from Chad a little about his past, the meaning of the coin and the vow he made. However, as the gang members fled they left a cell phone with an ongoing call to Yokochini, allowing him to spy on Chad and learn what might get a rise out of him. Yokochini later had Chad abducted and threatened him with the destruction of the coin. Ichigo stuck his neck out again, this time to protect not only Chad, but also the coin he held so dear. When the gang was defeated, Chad and Ichigo made a pact to fight for each other and protect one another.
Because they were constantly involved with fights and brawls with thugs and delinquents, both Ichigo and Chad gained the reputation of hardened thugs themselves. Their reputation far exceeded that of violent youths as they were made up to be juvenile gangsters of sorts. On their first day of high school, Chad and Ichigo were forced into pushing back a gang of thugs led by Reiichi Ōshima, who was hell bent on proving his bravado by beating up such a menacing duo. When Mizuiro Kojima befriended them, Reiichi started bullying Keigo Asano, forcing the two to help him and beat up Reiichi.