The day before hope was lost; -(from left) Zhang's father, an UIUC official, Zhang's aunt and boyfriend marched in Zhang's name on June 29, 2017. Christensen was arrested the next day. [Photo provided to China Daily]

In October last year Zhang Xinyang married and had a son, whom his parents are helping him take care of. "For them, the newborn may have provided some rare respite, but the pain inevitably comes back," says Zhang, who holds on to his sister's letters and diaries for consolation.

"Yingying is always present through her absence."

Barely two months after Hou returned to China last year he began his two-year stay in a village school tucked deep in the mountain wrinkles in Guangdong province, not far away from Zhang's hometown in neighboring Fujian province.

"Both Yingying and I had volunteered to teach during our holiday breaks at university, but we never got a chance to do it for as long as we would have wished," says Hou, who has taken with him several notebooks filled with Zhang's neat handwriting, notes taken by her in preparation for her studies in the US.

"None of the rules supposed to be followed by a small-town girl worked for Yingying. She was curious, fearless and relentless in her quest for knowledge-the making of a true academic."

The overwhelming majority of Hou's students are children left behind by their migrant-worker parents. Struggling with negligence and sometimes the insensitivity of adults around, many are in various states of mental anguish, some suffering from depression.

"What I had been through gave me the ability not only to sympathize, but to empathize with them," Hou says. "They make me aware that despite the hard carapace that has taken shape over me as result of what had happened, I'm still soft at heart."