Okay, to make it clear: Radu is not really evil!

Radu was Ion's best friend and guardian. He was sent to Caterina's place with Ion to protect Ion just in case. But he betrays them, and causes an explosion causing Tres to think Ion did it, while Ion doesn't understand why Tres shot him and runs.

Later they find out the RCO is using Radu's body to get close to the Empress and kill her