Among the photos that covered his mother’s coffee table are snapshots of Navy Hospital Corpsman 3rd Class Michael Vann Johnson Jr. the way his family remembers him: grinning in Mickey Mouse ears and waving at the camera.

“He was just a big kid,” said his sister, Janisa Hooks. “Mikey was a fun person. He liked to draw and he loved basketball, a real people’s person.”

Johnson, 25, was killed March 25 when he was hit by shrapnel while tending to injured colleagues.

He was raised in Little Rock, Ark., and graduated from the University of Central Arkansas. He and his wife, Cherice, lived in San Diego, where he was assigned to the 1st Marine Division.

Shortly before his death, his mother...