The Parsons Green Tube bomber was a "lovely lad" and "a bright and intelligent child", his foster mother has said.
Speaking in an ITV interview, Penny Jones said she felt "very betrayed" by Ahmed Hassan, who was convicted of attempted murder last week after trying to bomb a District Line train.
She added: "There is a small part of me that cares about him because I know he has mental health issues".
Hassan will be sentenced this week.
Mrs Jones and her husband Ron, who have fostered hundreds of children over five decades, gave Hassan a home after he arrived in the UK as an asylum seeker from Iraq.
They said they had no idea about the teenager's plan.
'In my house'
While the couple were away, Hassan made 400g of explosives and packed the device with nearly 2.2kg of screwdrivers, knives, nuts and bolts.
Mrs Jones added: "I am cross with him for what he's done, and that he's done this in my house makes me feel very, very betrayed, I can't help that.
"Because he's such a bright and intelligent child, he'd got a good future ahead of him. I find this hard."
In court, the jury heard that he wanted to avenge the death of his father in Iraq and was "disappointed" when the bomb only partly detonated.
Mr Jones said he still struggled to accept what his foster son did. "He was so convincing and I didn't have no cause to be suspicious of anything he had done," he said.