Raul Moncada enrolled in the Marines at age 17 — less than a week after graduating from high school. He was honored at one point as the youngest active-duty Marine in the country, his family has said.

“He was very outgoing. He would do anything he could for anybody,” said his sister, Miriam. “He was a great person all around. Everybody knew him. Everybody loved him.”

Moncada, 29, of Madera, Calif., was killed by a roadside bomb April 13 when his convoy came under attack as it was returning to a base in Baghdad. He was on his second tour.

“He was such a one-of-a-kind person,” said Cecilia Herrera, a longtime neighbor. “He was a polite person, a humble person.”

As the third-oldest in a family of eight children, he served in the Marines for six years before taking a one-year break. He...