To unlock the Pro Gamer you need to be playing as one of the Korean characters - Seoul Chicken, Jindo, Kimchi, Seonbi, Taekwondo Master, Korean BBQ, Korean Drummer, PSY, or K-Drama Actor.
Your goal is to get a score of over 40, while maintaining a really fast input speed. That means you need to avoid waiting around, move left and right if you can't go forward, and generally hammer the screen whenever you see a clear path.
You'll see how this all works when you unlock the pro gamer, who actually shows you your APM (actions per minute) on screen.
Ace (Formerly Jughead)
For this mystery character you'll first need to buy or unlock Rocky (formerly Archie), and then start playing as him. As you hop along you'll notice burgers along the side of the road. Land on one, to eat it.
You need to eat 50 (across as many different attempts as you want). When you hit the magic number, you'll get Ace upon your next death.
I'm guessing this guy is supposed to be Michael Bay, which means we'll need to make a massive great explosion to lure him into our game.
You can be any character. You just need to hop along until you see a big green truck carrying a rocket on its back, like this:
Hit into the rocket and it will explode into a stunning voxel-art eruption. You'll die, obviously, but you'll get a new character for your trouble.
This guy is pretty easy to get. Just unlock and use one of the British characters (that's butterfly, badger, fox, squirrel, Queen's guard, bobby, piper, chimney sweep, fancy lady, fancy gent, schoolgirl, or fish 'n chips).
Then, hop your way to the 100 point mark and you should reach a clearing with a phone box in it. Keep going until you die, at which point you'll add this outdated red cuboid to your roster.
This Scottish monster is basically like the Hipster Whale, in that she will be found swimming along a river and can be used as a platform to cross the stream. The only catch is - you need to be playing as a UK or Irish character. See the list, above.
You'll just have to keep trying. And trying. And trying. And hopefully you'll get lucky and have Nessie randomly swim across a river. If you see her, hop on her back and you'll get this mythical creature for your roster.