Stacey Rose Leondis ’08 had survived two bouts with osteosarcoma before her oncologist diagnosed her in 2006 with leukemia, caused by her chemotherapy treatment.

But the cancer did not deter her from what she did best: helping others. Leondis co-chaired Yale’s Relay for Life event the next year, and despite taking a yearlong leave of absence after her junior year during her second episode of osteosarcoma, she came back to school to finish her credits and graduate in May 2008.

Leondis passed away Monday, July 21, from complications due to the stem-cell transplants to curb her leukemia. She was 23.

Friends and family knew her for her ability to fight for those around her quietly.

“Aiding people and supporting people — that is really one of the defining things of Stacey’s character,” her father, Joseph Leondis, said.

Stacey Leondis was born on Feb. 11, 1985 and showed signs of brightness at an early age. She started playing the harp at age five and played soccer, lacrosse and field hockey throughout her childhood.

In school, she was “the most friendly” person you could meet, wrote a classmate from Garden City High School in The New York Times’ online guestbook.. “Our whole graduating class was friends with Stacey, no matter who you were.”

And she was the popular girl in high school, winning prom queen her senior year of high school.

But by age 16, Leondis had already halted her normal teenage routine, which included serving on the girls’ varsity lacrosse and field hockey teams.

Leondis and her family traveled to Memorial Sloan-Kettering Cancer Center in New York City, where an oncologist diagnosed her with osteosarcoma in the knee. She started chemotherapy treatments. She told the News in 2006 that the chemotherapy had a “marked effect” on her physical abilities.

Soon after the diagnosis, Leondis founded the FOSTER Foundation, which aims to stimulate osteosarcoma research, in her hometown of Garden City, N.Y.

The FOSTER Foundation — “Fighting Osteo Sarcoma Through Everyday Research” — serves as a source of research funding for scientists who study osteosarcoma, a disease that primarily affects children and the elderly.

FOSTER hands out $300,000 to $500,000 in research grants every year, according to Joseph Leondis, also a trustee of the foundation.

Leondis was a strong student, earning the fifth-highest score in a state middle-school math competition and later, in high school, the AP Scholar Award with Distinction for scoring well on the college-level exams.

Her academic excellence and her work in supporting cancer research were among the credentials that helped her gain entrance to Yale, where she started in the fall of 2003.

Leondis had pride for both her residential college, Saybrook College, and her university. During her senior year, Leondis and her mother, Ellen, bought all the residential-college memorabilia that Saybrook had in stock. She was a member of Colleges for Cancer, the Yale College Student Tea Club and the Skull and Bones society.

“It’s hard to imagine a more vibrant person than Stacey Leondis,” Saybrook College Master Mary Miller wrote in an e-mail. “Fun, energetic and, despite her struggle, the life of the party.”

“She’s always been a loyal friend,” Joseph Leondis said. “Stacey aided [her friends] in many, many different ways.”

Leondis made sure her friends relaxed just a little on the weekends, if possible, according to friend Sherry Wang ’07. After lunch on Sundays her sophomore year, Leondis would gather all student publications located outside the Saybrook dining hall and bring them to the room, so that she, Wang and other friends could sit around, read and gossip. They read and talked for an hour so they could take a break from studying and working, to “put our lives in perspective,” Wang said.

Soon after learning she had leukemia in 2006, Stacey Leondis worked with student groups to host a bone marrow drive at Yale. She did not take any credit, and she told the News at the time that it was a collaborative multi-group effort.

The drive, which occurred in September 2006 behind the Yale Bookstore and at the School of Medicine, attracted dozens of students.

At the same time, she provided initial funding through the FOSTER foundation for the “Back in the Game” program at the Professional Athletic Performance Center in Garden City. The annual program lasts 12 weeks and provides physical therapy and rehabilitation services to support children who have lost athletic ability from cancer.

After a year of absence to take stem-cell transplants that successfully defeated her leukemia, Leondis returned in 2007 to finish her senior year.

Majoring in biology, she took a multidisciplinary cell-system-modeling class her senior year and performed a thesis project in neurology. She was accepted to Mount Sinai School of Medicine for the class of 2012.

Although she could not attend her graduation, she still wanted to be a part of the graduation festivities. She donated at least $100 to the class of 2008 Senior Class Gift, according to its Web site. Miller gave her an academic award during the ceremony, her mother Ellen Leondis said.

“Did [cancer] impact her life? No question,” father Joseph Leondis said. “But was it the major thrust of her life? By no means.”