Jeffrey was an original member of the Indigo League upon its formation in 1946. He is sometimes called "the first of nature-lovers" by historians. He was the fourth trainer ever to be enshrined within the Hall of Fame.

Sanders was born in Cerulean City, a small town at the time. He lived in the western part of town, where as a child, he witnessed the damming of the Cerulean River. He grew up on the land that now belongs to renowned computer whiz Bill. He attended college at Cerulean State University, where he looked to acquire a degree in biology. He joined the battling club during his time there and began to enjoy it so much, that he continued to battle there even after he graduated. It didn't last forever, unfortunately, as the Great War soon dug its influence into him. In the May of 1942, he dropped out and joined the navy. He was finally discharged in May 1946, a full four years later, at the rank of corporal.

After the war, many veterans were left in poverty. Jeffrey was no different. He looked for work immediately after returning home. His father had become disabled due to a work-related incident, leaving his family without a stable income. For this reason, Jeffrey fought through his minor case of PTSD in order to provide for his family. Upon looking for work at the Cerulean dam, Jeffrey lucked out when he met the owner, Indigo Misaka.

"He looked like an awkward man in a way, but once he spoke... He sounded like a man who had seen the most terrible things this world had to offer... And gave the impression that those things didn't even make him flinch." Misaka later said in an interview for the league's 25th anniversary.

Misaka offered a position as a gym leader in the league he would be founding within the next few months. This came after Jeffrey mentioned his PTSD and family situation. He declined Misaka's offer at first, not wanting to accept pity from Misaka whatsoever. "I had to convince him it was out of the kindness in my heart, and that my intentions were genuine." Misaka recalled with a chuckle. Jeffrey soon accepted, and became he league's second gym leader.

Jeffrey embraced the Bug type due to one of his war experiences. On one operation in particular, a sputtering engine in his patrol boat cause him to be marooned and stranded upon an island in the middle of the ocean with three other sailors.

"Day 5... We're still alive and well. We've been playing cards to keep us occupied. The bug Pokemon here keep us safe, though. I've seen a Scyther ward off an Ursaring that attempted to raid our campsite. The bugs here are far from pests... Neither are they pets. They are... Friends. At least in my eyes. The Leavanny are my favorite. I've never seen them before, but they are adorable. They remind me of dolls I would help my dad carve out for this girl I liked in school. Who knew creatures in this wild could be so hospitable? The Butterfree hover about us in the day while the Venomoth do the same at night, always to make sure we are okay. This has been one of the best experiences of my life, that's for sure."
- Jeffrey, from an audio recording during his island stay.

Data from specific battles of Jeffrey is scarce, but we do know plenty from what surrounded his life. Jeffrey would host battles just outside his home, where the Cerulean Cape is now located. Rather than big sporting events in style like the other leaders, Jeffrey preferred a more laid back approach.

"Why stress about life? The main advantage we have to fight stress is the ability to choose one thought over the other. So why stress about life? Focus on positives and the beauty of life will come your way."
- Jeffrey, 25th Anniversary Interview, 1962

Jeffrey would often invite people to relax at his battles. Some would say that his battles looked like family barbecues from a distance, with the way people sat and were strewn about with their causal lawn chairs. Everyone went to relax and have a good time until they were too tired to go on.

His battling technique was also very unique. "He would barely talk during battles. The way he would just sit there with his legs crossed and eyes closed, a grin on his lips... It intimidated me a bit." Former gym leader Adrianne said of her colleague.

Jeffrey did not talk during battles much. He trusted the thought process of his Pokemon, and knew they could handle situations. You would only see Jeffrey stand and shout orders during the gym leader tournament at the end of the battling season. "He was a competitive guy, and wanted to win badly." Adrianne said of him.

From his entrance at age 33 in 1946, to his eventual retirement in 1985 at the age of 70, Jeffrey was a prolific battler. He retired with a career winning percentage of 62.3%. Despite his competitive drive, Jeffrey only acquired one tournament championship, the first one overall in 1947. He came in as the underdog 7th seed in every matchup. Being labeled as an underdog drove him to succeed against his fellow gym leaders, no matter the odds.

However, he failed to return to the Indigo League Finals over the next ten years. When he finally did in 1957, he lost to Adrianne herself, whom of which earned her sixth championship. It was a matter of youth in the eyes of the spectators. Adrianne was in her prime, matured fully at the age of 31. Jeffrey had a family now with two kids. First signs of aging were showing on Jeffrey's face, between wrinkles and occasional gray hairs.

The town, however, stuck beside him. He was the one responsible for all the plant life around the city, after all. He made sure Cerulean Cave was what he deemed "pure" with wildlife roaming freely without interruption.

The yearly losses in the tournament drove him insane, however. Not to the point where he was hostile, but very unstable. His competitive ego couldn't handle the losses at all.

"They say the definition of insanity is doing the same thing over and over again while expecting a different result... I think I've reached that point, I would say. I'm not happy with just one... The way faces lit up here in town when I won that first one... I wanted to see it again. My dad was watching when I won it all that year. He was proud, for sure. He applauded me but encouraged me to better myself, as he always did. "Go out and get another", I remember him saying. I'm sorry dad... I failed you..."
- Jeffrey, Hall of Fame Speech, 1986

"It made me emotional when I saw him like that on the podium. I knew at that moment there would be another Bernarda, another Malcolm, even another Adrianne... But there'd never be another Jeffrey Sanders."
- Bernarda, Yearly Interview, 1987

Upon his retirement, Jeffrey decided to return to his alma mater of Cerulean State University. He taught students about the environment, and about psychology. He also coordinated intramural battles within the school.

On March 21, 1989, at the age of 74, Jeffrey was shot and killed during an argument over a handgun with his son-in-law, Steven Bonner. The argument started when Jeffrey's daughter, Delilah Bonner, complained of being uncomfortable with her husband having a protective handgun in his closet. Other details of the case are carefully and mysteriously hidden away by the Cerulean police to this very day. Handguns are also now banned from the city of Cerulean as a result.

In 1990, a ship in the Cerulean Naval Yard was dubbed the "S.S.J.S.", in honor of the former corporal. Soon after, the ship was decommissioned and built into a museum on the environment, with one of the wings dedicated to Jeffrey and his life. The original plan was to bury him upon the boat or at sea, but at the request of his wife Sandra, the idea was soon revoked. He is buried in a tomb within Cerulean Cave, which is rumored to be protected by two giant Pinsirs.

If one were to walk into Cerulean State University today, they'd see a large banner above the battle arena in a silky lime green, his name printed in gold upon it. Underneath his name is a silhouette of a Beedrill.

In 2006, computer whiz of Kanto, "Bill", purchased Jeffrey's now abandoned home for a grand total of 500K in Kanto currency. Bill reportedly hired a channeler from Lavender Town to purify the home a few weeks after. His reasoning remains unknown.

In 2010, Cerulean Gym Leader Misty lobbied for the town to craft a statue or memorial in Jeffrey's honor. The lobbying fell through however, and Misty instead settled for renaming her gym building as the "Jeffrey Sanders Battle Center."

The 1947 championship banner still hangs with pride in Misty's gym, a clear reminder of the Cerulean man who fought his heart out to obtain it.