The sixth mystery character is Andy Sum. In order to unlock him, you have to get a Great Score with the Mallard, which is his favorite character. After that, you’ll unlock him and he will become one of your playable characters.
The seventh mystery character and the last of the developers is Matt Hall. He, just like Andy Sum and Ben Weatherall, is a developer of the game, and the way that you unlock him is to use his favorite character and earn a Great Score. In this case, his favorite character happens to be the Lucky Cat.
So how do you get a Great Score? This is the subject of a lot of controversy, but the way to do it is to either earn 150 points in one shot, or to come within 50 points of your top score, whichever is greater. So if your top score is 118, you have to earn 50, but if your top score is 218, you have to earn 168 in order to make it happen.
As for the crab, which tends to be a severe pain to get, try to get at least 50 points, and stop to go side to side as fast as you can at every sand area. Once you do that, then die, and you will get the crab. Use a different Australian character if it doesn’t work; unlock more characters if you have to.
Thanks to the youtuber CraniumCodes as well as you, the commenters on WriterParty, for the good info about most of these hidden characters. Read on for more of the secret characters; new ones are being added all the time. As of the time of this writing, there are eight.
One note. The Disco Zoo character, the elephant that serves as the logo for the Disco Zoo game, is LOADED with easter eggs; however, this is not a hidden character so there is no need whatsoever to do anything to unlock the character that’s any different from what you would do for most others. Spend 100 coins at the Gacha/gumball machine/whatever and hope that it shows up.