Wo Kao Mei Yan Wen Zhu Tianxia

Gu Yuanbai transmigrated into a boy's love novel as a side-character, the sickly and weak emperor. He tried to make something of it, but it turned out he only had a few years to live anyway. So he decided to sit back and watch the main characters, the aggressive regent Xue Yuan trying to seduce the beautiful courtier Chu Wei. But Xue Yuan's tactics were too over the top, and Gu Yuanbai decided to take him down a notch. But as Gu Yuanbai tried to bring the mad dog Xue Yuan to heel, he found that his meddling is making himself stronger, and causing the plot to go off course...

(Source: Kuaikan, translated)