The incident occurred around 11:32 p.m. in the 2200 block of Lewis Avenue in Long Beach.
According to family members, Eric Brown was hanging out with friends when shots were fired.
Brown was killed while the teen girl was injured. Police said she sustained non-life-threatening injuries to her lower body. A third teen, another young girl, wasn't hurt.
"Who shoots kids? I mean, why are they shooting kids?" asked Brown's grandmother Cynthia Johnson. "He wasn't no gang-banger or no nothing. He was a child sitting outside on the steps."
Police believe the victims were walking on the sidewalk when the suspects, believed to be two men, approached them in their vehicle.
Family members said the teens were reportedly asked "Where you from?" before shots were fired.
"I just wanted to say whoever did it, can you please just turn yourself in, please," said the boy's mother Angela Washington.
Brown was the second of five children. During a press conference Wednesday afternoon, the boy's older sister shared what she loved most about her little brother - all while holding back tears.
"He always made me happy," she said. "He always made laugh. I love being around Eric. Eric was a good person to be around. I love his personality. I love everything about Eric."
Long Beach Police Chief Wally Hebeish called the shooting "absolutely devastating."
"A young boy's life has been cut short due to an intolerable act of gun violence," he said. "While we mourn this loss as a community, our officers will not stop working to locate and arrest those responsible for this tragedy."
Meanwhile, Brown's family is struggling to accept their new reality.
"My grandson was 12 years old, an innocent 12 years old, and I just can't understand how anyone can be that evil and senseless to kill a minor," said Sonia Age. "He looked like a minor."
A description of the suspects has not been released.
Anyone with information on this incident is urged to contact authorities.