“Speak the truth even if your voice shakes,” Khaseen wrote.

His family moved to Oceanside this summer from Freeport, a neighboring town on the South Shore of Long Island. There, Khaseen saw the promise of a fresh start at a school where he could fully express himself through fashion and art, said his sister, Kedeemah Morris, 22.

Image Khaseen Morris

He was a free-spirited skateboarder who wore his dreadlocks colorfully: half orange, half black, Ms. Morris said in an interview on Tuesday, adding that he loved to draw and that he had planned to study photography.

The siblings got the same tattoo, she said, in memory of their favorite musicians, Lil Peep and XXXTentacion, both of whom sported bi colored hair . Ms. Morris said she and her brother were so similar that she called him her “twin” despite their six-year age difference.

“He danced, he was very funny,” she said. “He was very sweet.”

On Sunday, she said, a 10th-grade girl asked her brother to walk her home and he obliged. Ms. Morris said she believed that the girl had been trying to make her ex-boyfriend jealous.

It appears to have worked.

Before Khaseen left for school on Monday, he told his family that he had received a threat from the girl’s ex-boyfriend, Ms. Morris said.

“My brother, he shrugged stuff like that off,” she said. “He told the boy he didn’t want his girlfriend, it was nothing like that, he just walked her home. The boy said he didn’t care.”