Takeru

Takeru is an unseen character who is mentioned in Golden Sun: The Lost Age and Golden Sun: Dark Dawn. He is the oldest son of Susa and Kushinada. His Encyclopedia entry reads as follows:
 * This courageous warrior is the son of Susa and Kushinada, making him the prince of Yamata and heir apparent. After his sister, Himi, had a vision of the world in peril, he went abroad on an important mission.

Story
Takeru's name is first brought up by Kushinada after the Serpent has been slain. When targeted by Mind Read, Kushinada will mention that her future son with Susa will be called Takeru. She plans to tell him about Felix's adventures, and according to her, Takeru will rush to Felix's aid when he is needed.

In Dark Dawn
Takeru is the Prince of Yamata, and is stated by several citizens to be one of their strongest warriors. Following a premonition from his sister, Himi, Takeru leaves Yamata in order to help Isaac. This occurs prior to player's arrival in Yamata, and thus, he is not actually seen in-game in person. Despite this, it's likely hinting the possibility that he becomes a playable character or at least will be finally seen in person in the next Golden Sun game.