Krillin is available from the start. His DU is one of the more lengthy ones, it is only shorter than Goku's by a little. You will have to play through his DU just under two full times to unlock all characters you need for the completion. In your first run, just race to the end, as there is nothing different you need to do. In your second run, after the fight with Frieza, two red dots will appear. Go to the one on the right, and you will enter a scene with a frog. Continue, and your story willl progress to the Android Saga rather than finishing. When this section starts, go to the area near the world tournament, and search for a "???". You will find Android #16, but he will be broken. Now go to West City, and you will unlock him. Once this has been done, you can quit out of this DU.