Here is a list of the Regional Arena Defender's in the game also known as RAD's. They are in order of completion in the game with all details needed. Defeating these RAD's earns you the Complete Zenith Key, which gains you entrance to the Coliseum at the end of the game. They are the main objective of the story, along with saving your father.
All RAD's Creo levels are scaled except for Odlare (The higher your Creo level, the higher RAD's Creo level).
Odlare is the 1st RAD within the game. He is located in Plantae Village, to the right of the arena pillars, between the barn and a rock. When you beat him, he will give you a choice of choosing between one of the Creo in the barn. You can choose Monkopod, Skalanka, or Fyroey
Gravare is the 2nd RAD within the game. He is located in Carbon City. As you enter from Largo Bridge, head up and he will be standing to your left, looking out the cliff. He is affectionately referred to as "Mud Daddy".
Ganzo is the 3rd RAD and is located in the top left corner of Electron Metro. Stand outside the Evoco and go left a little, Then head up between the tall building and the EvoCo, turn left and walk all the way into a woods area. You will pass 2 NPCs then go up a little. He is standing just behind a rock, left of a tree. Evidence points to him owning a collection of Squirian body pillows.
Saleh is the 4rth RAD. He is located after the Cave of Wonders. After exiting the cave, follow the path across the bridge and through the Evokers. After the bridge, he will be on your left near the sign looking at a tree. He is the best dressed of all the RADs.
Eldare is the 5th RAD and probably the hardest to find. Use Fly at Trail 21, then when in the sky, follow the path slightly left and then head down. This path is long and will go up and down the sky with Evokers and items. Follow this path until you reach the Volcano. You will be on the top floor of the volcano. Use lava surf and head down and right to find him on a small island. A notable hobby of his is setting bystanders on fire.
Reina is the 6th and final RAD. She can be found in the Cave of Winds to the North of Atmos City. Once inside the first cave, follow the path to the left. When it comes to a T, continue going right. Eventually, this should lead you to the second part of the cave. Once in the second part, make another left, and go down. Make a slight left the next time you can, and Reina will be on the platform in front of you. Reina has once attempted to fly herself, resulting in 2 broken bones and a missing finger.