To get the true ending is a long process. First you have to go to the beach and check the ice cream stand. After that collect the syringes and go back to the road. Then, go to the Fun fun fun guy and collect samples from him and accept his quest. Next, Go to monke HQ and collect the glowing key. Go to the mysterious man in the basement and use the glowing shard on him. Collect samples after he melts. Finally, go to the robot and put both the samples in and press the green button. The rest is up to you.
