By Brad Poole

TUCSON, Ariz (Reuters) - The two best friends of the youngest person killed in a hail of bullets outside a Tucson supermarket last year recounted on Sunday how the three girls shared a secret pact to attend the same college when they were grown.

In a day filled with prayers, tributes and tears marking the anniversary of the shooting rampage that left six people dead and 13 others wounded, including Arizona Congresswoman Gabrielle Giffords, the remembrance of 9-year-old Christina-Taylor Green was one of the most poignant.

Christina's two closest friends and classmates were the final speakers to address more than 1,000 people who packed an auditorium at the University of Arizona at Tucson to hear 90 minutes of reflections honoring the victims of the attack.

Recalling the details of a childhood cut short by tragedy, Serenity Hammrick and Jamie Stone spoke of how the three girls attended cheerleading camp together, sang Hanna Montana songs together and passed idle time daydreaming.

"Sometimes we would just go to the park and lay on our backs and just chill and look at the clouds," Jamie said.

Their bonds ranged from the playground to their plans for college.

Christina, granddaughter of former Major League Baseball manager Dallas Green of the Philadelphia Phillies, joined her two pals in secretly promising each other that they would all go to the University of Arizona together, her friends said.

Serenity also remembered Christina helping her overcome her fear of playing kickball at school.

"Christina encouraged me by letting me practice with her. She wasn't afraid of boys or sports or anything," she said. "Sometimes it seemed like she was the only one on my side."

She added: "I want everyone to understand that Christina was one in a million, and she was my best friend."

Serenity recounted a special day the three girls spent swimming in a local creek and talking about their futures.

"She said something that day that I will never forget. She said, 'You aren't my friend, because you're my sister,'" Serenity recalled.

(Writing by Steve Gorman; Editing by Chris Wilson)