Zhaoju
Here's a possible start to your story with the added details about the protagonist's background and experiences:
Liang, a name meaning "bright" or "shining" in Chinese, was a fitting irony for a boy who'd transmigrated from Earth, where he'd died as a nameless, family-less orphan beggar on the streets. In this new world, his life began miserably, but he formed bonds and friendships along the way. At 12, he traveled to the Revolutionary Kingdom to become a mercenary, but his frail, slender build and beautiful eyes made the mercenaries doubt his suitability. They gave him menial tasks, but it was during this time he met Mr. Bai, the most wisest man acknowledged in the three kingdoms for his military strategies. Mr. Bai saw potential in Liang and taught him military strategies and formations.
Liang's heart, however, was with the princess of the Huahi Kingdom, whom he missed dearly. In the Huahi Kingdom, despite his intelligence as a transmigrator, he'd been bullied by children from the 12 steppe tribes because of his tender, frail appearance - much like the horse he chose, different from the bulky, strong ones others preferred. Later, the mercenaries he wanted to join tricked him and sold him as a slave to the Balak Kingdom. There, he met his future lifelong rival and friends. His friend managed to escape, but Liang got rescued by the most feared and notorious bandit, whose power was already approaching that of a small kingdom.
Now, at twenty, Liang stands at a crossroads in a world of darkness - wars, diseases, slavery, and natural disasters. With Mr. Bai's teachings and his own transmigrator's mind, he has the potential to leave a mark. But with his heart tied to the Huahi princess, and his experiences of betrayal and slavery, what path will he choose?