Along with Castillo and one other student, Brendan Bialy, an 18-year-old student at the STEM School in Highlands Ranch, Colorado, was able to disarm one of the alleged shooters and pin him to the ground. Castillo, 18, was killed as he charged at the accused gunman and threw him up against the wall.
“There was no questioning. There was no hesitation. There was no looking around,” Bialy said of Castillo as he spoke to reporters at a news conference.
“It’s really hard to stop that kid when he gets going,” Bialy said, likening Castillo to a human bowling ball as he rushed to overpower the suspect. “The gunman … was against the wall and didn’t know what the hell hit him.”
John Castillo, Kendrick Castillo’s father, called his son’s death “devastating, as you can imagine.”
“When I see the people that he saved, it made me happy,” John Castillo said. “I knew my son wouldn’t have it any other way. But as any parent would tell you, ‘It’s a heck of a trade off.'”
Bialy said he had known Castillo since their freshman year and said they had been close friends. He described Castillo as a “fantastic, wholesome person” who loved cars and electronics.
Bialy was seated on the far side of his classroom on Tuesday afternoon when one of the two alleged shooters entered the room brandishing a pistol. He said his fight-or-fight response kicked in after seeing his friend stand up to the threat.
Bialy was able to get the gun away from the suspect while the third student, who has not been identified, joined in to help and pinned the suspect to the ground, Bialy said.
“He did what he had to do and that was my son’s nature,” John Castillo said. “That’s what he does.”
Bialy said the gun went off once or twice. After Castillo was shot, an IT teacher came into the room and began performing chest compressions on the 18-year-old.
Castillo later died at a local hospital. Eight other students were injured in the shooting, including some in that same classroom, authorities said.
The only injury Bialy received in the scuffle was scrapes on his knees.
“I was blessed by something,” he said. “Somebody’s watching down on me. Something, somebody, I don’t know. Even though I was inches away, I didn’t get shot.”
Bialy described the gunmen as cowards. He said he knew both suspects, including one who was in his graduating class, but he only saw one of the suspects in his classroom. Other students who were in the same room were also shot and injured, he said.
Both suspects appeared in court Wednesday afternoon. The adult suspect, 18-year-old Devon Erickson, has been charged with murder. The other suspect, a juvenile, has not been identified, and any charges against him are not yet known.
When asked by a reporter on how he was “able to smile” despite having just experienced a tragedy, Bialy explained that it was because he got to see “the absolute best in people.”
“I got to see two heroes — two regular high school kids, two really awesome people — jump into action,” he said. “I was more than lucky to join them.”
Bialy, who is a member of the U.S. Marines Corps’ Delayed Entry Program, rebuffed any suggestion that he was a hero, but said he has received congratulatory messages from several fellow U.S. servicemen.
“I need to let them know that Kendrick Castillo and that other student were in the thick of it, rolling in the ground, where I was too,” he said.
“Kendrick Castillio died a legend,” Bialy said. “I know he will be with me for the rest of my life.”
John Castillo began to cry as he remembered the time he spent with his son.
“He was always going to be with me man,” he said. “He’s always going to be here. … We were friends first and his parent second and I love that.”
ABC News’ Sabina Ghebremedhin contributed to this report.