Radio is a companion that is given to Luka, she is a parallel world's last functioning Makina.
Luka and the party have ventured through the third Tartarus and entered another dimension. Within this dimension, the world itself is being consumed by chaos. Both Sylph and Gnome inform Luka that their corresponding elements are dead.
At the entrance to the tower, the party is greeted by Radio. She is currently serving as La Croix's research assistant. She fills them in on the current state of the world: it's ending. The last human died 25 years ago; the last angel died eight years ago, and the last monster died six years ago. The only building still standing is this tower. Radio offers to lead the party to La Croix, on the third floor. She asks them to be careful as La Croix has created artificial monster girls to fight off apoptosis beings, and they will likely attack Luka.
By the time the party reach the top floor of the tower, there is nothing left. Just Radio, La Croix and the Cirque du Croix.
La Croix explains to Luka that their world will soon cease to exist due to the spread of chaos, and asks Luka to take Radio with him back to his world. She's grown fond of the machine girl and doesn't want to see her broken. Luka tries talking La Croix into coming with them, but having two La Croix's in the same world would increase the rate of chaosization, and refuses.
"Today is another good dAy. I am Radio, alsOKnown as a piece of junk."