Takeru

Takeru is Susa and Kushinada's son. In The Lost Age, Kushinada, when targeted by Mind Read, mentions that her son will be called Takeru and will rush to Felix's aid when needed.

In Dark Dawn
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, but is mentioned in conversation. 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.