Jayce is Urf in disguise.
You see, when Warwick killed Urf, his spirit was still in this world. It wardered around, searching for a way back, when it found Jayce. Jayce, at the time, was still working on his Mercury Hammer. As Urf was wandering nearby, Jayce accidentally put his blowtorch up against the nuclear core of the device, exposing himself to a lethal dose of radiation. As he lay dying, Urf took pity on him, and invigorated Jayce with his spirit energy so he could live. Now, Jayce and Urf live as one being. Jayce originally wanted to use the hammer as a weapon, but Urf had other plans. He convinced Jayce to keep the hammer and the blueprints of the hammer to himself and, using the power of the hammer, join the League. Little did Jayce know, Urf was plotting his revenge on the one who took his body away.
And that is why Jayce joined the league. Urf is still plotting his revenge to this day.