Rob Kardashian and Blac Chyna took their relationship public last week
Credit: Alexander Tamargo/Getty Images; Jeff Vespa/Getty Images for The Hollywood Reporter

Khloé Kardashian has always been her brother’s biggest advocate throughout his struggles with depression and extreme weight gain, inviting Rob to live with her and defending him against her famous family’s attempts at tough-love interventions.

“Rob is such a great guy,” Khloé, 31, told PEOPLE in October. “He’s charming, charismatic, everything. That’s what’s so infuriating. I’m like, ‘Where is your will to live life? Let’s be around people!’ ”

So it hurt the reality star when the person who seems to have given the push he needed to kickstart a healthy lifestyle turned out to be a Kardashian enemy: Blac Chyna, who was once engaged to and shares a son with their sister Kylie Jenner‘s boyfriend Tyga.

In this week’s issue of PEOPLE, an insider says Khloé felt “betrayed” by the blossoming romance.

“She has gone out of her way a million times to help him,” the source says. “She always defended him.”

For more on Rob Kardashian’s relationship with Blac Chyna, her arrest and how the Kardashian-Jenner family is reacting, pick up this week’s issue of PEOPLE, on newsstands Friday

Rob, 28, and Chyna, 27, took their romance public last week and have been demonstrating plenty of PDA on social media ever since.

As for Khloé, “She was frustrated by his lack of respect,” the source says.