This NBA star didn't want his transgender sister at his games. Then he lost her.

NBA star Reggie Bullock struggles with a big regret.

Photo by Nick Laham/Getty Images.

When he was younger, Bullock didn't want his sister, Mia Henderson, to go to his basketball games. Henderson was transgender — and Bullock was ashamed.

"I always thought about what my teammates would think or what people would think," Bullock, who now plays for the Detroit Pistons, explained in a heart-wrenching video shared by his team's Twitter account. "That was probably one of the stupidest things, but I just didn’t know so much about [being transgender], and I wasn’t as comfortable with it.”

Henderson was murdered in 2014. She never got to see him play in college.

Mia Henderson. Image via The Detroit Pistons/Twitter.

A lot has changed over the past four years. Bullock's made steps to learn about LGBTQ issues — particularly, the challenges faced by people like his sister.

Transgender women of color are especially at risk of violence and discrimination in employment and housing.

"It was something I didn't know too much about [when Henderson was murdered] — about the deaths and the things that happen to this community," Bullock said.  

In honor of Henderson, Bullock got a tattoo on his leg reading, "LGBTQ," along with Henderson's name.

GIF via The Detroit Pistons/Twitter.

He also wore "equality" sneakers during a game against the Atlanta Hawks in February to acknowledge the impact she made on his life.

Photo by Kevin C. Cox/Getty Images.

But Bullock wants to go much further than symbolic gestures.

Bullock has partnered with LGBTQ rights groups to amplify Henderson's story and shine a light on an often overlooked community.

The Pistons small forward hosted a Pride Night in Detroit to meet fans and talk about LGBTQ issues. He also synced up with LGBTQ rights group GLAAD and Athlete Ally, an organization aimed at combating homophobia and transphobia in sports, to use his platform to spread love and acceptance.

He may not know all the facts quite yet. ("Is it just LGBT?" Bullock asked advocates, wondering if the "Q" at the end was appropriate or not. "You can do either — LGBT or LGBTQ, both are fine," someone answered.) But he's committed to learning and making a difference along the way.

"Anything that y'all can put my face on or use me for — parades or stuff like that — I'm willing to do it," Bullock noted in the video below.

Photo by Kevin C. Cox/Getty Images.

"The fact that Reggie Bullock wants to use this as an opportunity to talk about his support for LGBT people, and especially for transgender women, is really incredible," said Nick Adams, GLAAD's director of transgender media and representation.

While Bullock believes he'd still be a champion for LGBTQ rights and visibility even if Henderson hadn't been transgender, her story is still at the heart of his drive to make change.

"The impact she had on my life and the happiness that she had with just being herself always stuck with me," he explained. "Even when she left.”

Watch the video below by the Detroit Pistons to learn more about Bullock and Henderson's story:

To learn more about issues facing the transgender community, visit GLAAD. To help combat homophobia and transphobia in sports, visit Athlete Ally.