Sister Chu indeed felt sorry for him at first, so every once in a while, when she couldn't stand by, she would lend a hand, sometimes a few old clothes, sometimes a few steamed buns. Over time, he indeed developed a sense of gratitude.
There were so few people who meant well by him that even the slightest kindness was enough to overwhelm the young Si Chu with gratitude, and he was even willing to pay a greater price for it.
But later on, the Noble Consort learned about Sister Chu's existence and the things she had done, so she enticed her with money and treasures, starting with some inconsequential conditions.
The human heart is the easiest thing to compromise.
At first, Sister Chu might have harbored pure goodwill toward him, but faced with great temptation, she eventually became tempted, especially since the Noble Consort had only set forth some simple conditions.
Using the trust he placed in her, time and again she reported his movements to the Noble Consort.