"Jake was never the violent type," Roberts' ex-girlfriend, Hannah Patricia Sansburn, told ABC News. "His main goal was to make you laugh, smile, make you feel comfortable. You can't reconcile the differences.
"I hate him for what he did, but I can't hate the person I knew because it was nothing like the person who would go into a mall and go on a rampage," she said.
The Clackamas County sheriff's office said Roberts had several fully loaded magazines when he arrived at the mall Tuesday. Roberts parked his 1996 green Volkswagen Jetta in front of the second-floor entrance to Macy's and walked through the store into the mall and began firing randomly in the food court.
He fatally shot Steven Mathew Forsyth, 45, and Cindy Ann Yuille, 54, the sheriff said. Kristina Shevchenko, 15, was wounded and in serious condition Wednesday.
Sansburn said Roberts had recently quit his job at a gyro shop in Portland and sold all of his belongings, telling her he was moving to Hawaii. He was supposed to take a flight Saturday but told her he got drunk and missed it.
"And then this happens. ... It makes me think, was he even planning on going to Hawaii?" Sansburn told ABC News.
Sansburn didn't return phone messages left by The Associated Press, and no one answered the door at her home Wednesday.
On a Facebook page that a friend identified as Roberts', a few photos show Roberts with friends, while one shows the back of a person in a knit cap firing what appears to be a handgun at targets. The cover photo is of a wall painted in graffiti with the message "Follow Your Dreams" and the word "Canceled" stamped across it.
In the public portion of his page, Roberts wrote: "I may be young but I have lived one crazy life so far."
"I'm the kind of person that is going to do what I want," he wrote. "There is no reason for another person to tell you what to do, I'm the conductor of my choo choo train."
He added he was "a bit of an adrenaline junkie" who was "just looking to meet new people and see the world."
A former neighbor of Roberts said he liked to play video games and never seemed troubled.
"We knocked on each other's door every morning. Every day to hang out, to talk," said Samantha Bennett, who added she went to middle school with Roberts but wasn't close to him until he moved in with a girlfriend across the hall from her at an apartment complex in summer 2011.
"If me and my boyfriend were fighting, he was there to talk to me," Bennett said. "We would go to the bar together. I don't get it."
Roberts' dining room was decorated like a jungle, Bennett said - just one of the quirky things he did. He had green lights and put ivy on the walls.
He once showed her a black handgun that she believed he purchased legally. He dropped out of sight earlier this year, and his phone was disconnected, she said.
More recently, Roberts rented a basement room in a modest, single-story Portland home and hadn't lived there long, neighbor Bobbi Bates said. She said she saw him leave at 1:30 p.m. Tuesday wearing a dark jacket and jeans, carrying a guitar case.
Police say Roberts had stolen an AR-15 rifle from someone he knew.
The first 911 call came in at 3:29 p.m. Officers arrived a minute later. Instead of waiting for SWAT teams, police immediately entered the crowded mall.
Police told people inside to put their hands in the air, to make sure an armed person wasn't among them. Police spent hours clearing the 1.4 million-square-foot mall, as some workers and shoppers continued to hide in fear.
Wearing a hockey-style face mask and dark clothing, Roberts fled along a mall corridor and into a back hallway, down stairs and into a corner where police found him dead from an apparent self-inflicted gunshot, authorities said.
Clackamas County Sheriff Craig Roberts said the fact that more people weren't killed was due to several factors. The suspect's gun jammed at one point; the mall implemented an immediate lockdown; and a large number of officers arrived on-scene quickly, "curtailing the suspect's ability to move around the mall."
"Ten thousand people in the mall at one time kept a level head. They got themselves out of the mall. They helped others get out, and there are just a number of heroes that took the time to help people get out," the sheriff said. "It was really about a full group of people coming together to make a difference."
Contributing to this report were Associated Press writers Steven DuBois, Jonathan J. Cooper, Nigel Duara, and Sarah Skidmore in Portland, Pete Yost in Washington and Manuel Valdes in Seattle, along with researcher Rhonda Shafner.