On Southern Island, when the player talks to Daikun in gameplay, he can assign Sonic missions. Should the player completes these missions, Daikun will decorate Southern Island as an reward.
For an unprecedented amount of time, Daikun has lived on a little, seemingly deserted, island. Sometime prior to the events of Sonic Rush Adventure, Daikun came to know Marine when she was very young.
While living on his island, Daikun heard weird sounds coming from an island to the west. Finding it too noisy, Daikun traveled to the island to find the origin of the noise, where he discovered that it was like the island itself was shaking or at least something else suspicious.
Later on, Daikum found Sonic, Tails, Blaze and Marine arriving on his island. Not pleased that the group had stepped onto his island, Daikum quickly confronted the group and demanded to know who they were and why they had come until he recognized Marine and calmed down.
After introductions had been made, Daikun apologized for his behavior by explaining that he did not know that they were with Marine. Seeing that Marine did not recognize him, Daikun assumed that Marine had forgotten about him, though Marine explained that she was too young to remember him. Having already mentioned about the island to the west, Daikun told the group about his own investigation of the island, and group decided to investigate it.
After the group thanked Daikun for his help and headed off, Marine offered Daikun to come back to Southern Island at some other time. Despite the circumstances, Daikun took up on Marine's offer and returned to Southern Island afterwards.
↑Setter said near the end of Sonic Rush Adventure that "Now that all the members of the Coconut Crew are back, the village is nice and lively again." At that point, Daikun, Colonel, Kylok, Muzy, Setter and Tabby are all present at the island. Excluding Marine, everyone else on the island (namely Sonic, Blaze, Tails, Norman and Gardon) is a foreigner, meaning that Setter most likely referred to the previously-mentioned group of koalas as the Coconut Crew.