Shawn Santo’s infectious smile and outgoing personality will be forever remembered by his family and friends, who are mourning the loss of the 20-year-old Valley Stream Central High School graduate.

Santo, who lived in Bellmore, died on Christmas Eve of apparent heart failure, though the exact cause has not been determined.

His family moved from Queens to Valley Stream when he was in junior high school, and he attended Memorial and Central before graduating in 2010. During those six years, he made countless friends. “He was an outstanding kid, a smile on his face every day,” said Principal Dr. Joseph Pompilio. “He lit up a room when he walked in. He knew everybody.”

Chelsea Starna, a fellow graduate, said she met Santo when they were in seventh grade, and that he was a little shy back then. But he eventually came out of his shell, and by ninth grade they were good friends. In high school, Starno recalled, they were in many of the same classes, and also spent a lot of time outside school together.

“Everybody really loved him,” she said. “He was friends with everyone.”

Starna said that Santo took the initiative to get to know people; he didn’t wait for others to come to him. And he had a lot of school spirit, she said.

Nicole Menna said she still remembers the moment she met Shawn — seventh grade, second-period English class. From there a long-lasting friendship blossomed. “Shawn was pretty much my best friend,” she said. “Now he’s my guardian angel.”

When they got older, the two would often spend hours just driving around, singing along to songs on the radio.

In their junior year, Santo and several classmates took part in the annual Breast Cancer Walk at Jones Beach. It was early in the morning and everyone was tired, and Santo — who had dyed his hair pink — made it his mission to liven everyone up, even giving his friends piggyback rides, Starna recalled.

Santo leaves behind an older sister, Stephanie, 30, and an older brother, Steven, 27. Their mother died when he was 14, and they lost their father two years ago.

Stephanie Santo said that she and her youngest brother were very close, and she helped raise him after the death of their mother. “He was my best friend,” she said. “I talked to him about anything and everything.”