Reincarnated as a Side Character with an Overpowered System RSCCS

Joshua was a hardworking youth who worked in a library. One day, he was a testimony in a robbery, and unfortunately ended up dead.
He then found himself inside the body of a youth with the same name as his, in a cultivation world.
But it wasn't that simple. Joshua was inside a novel he had read back on Earth, but he wasn't the MC but a side character.
At least destiny was kind enough to give him a cheat system to make up for not being the MC.
And not only that, the MC was still out there, with a system of his own, and that system was actually slowly trying to take over the MCs body.
What should Joshua do? How can he overcome the MCs heaven defying talent and discover what the MCs system wants at the same time?