The first step is just to main him in a maze. You should get an item called a “Classic Monster Illustration” at the start of an early floor (<15?) or from the first picture or two you take. You’ll need to settle the maze before you can do the next part.
After that there are four special photos you can take that will give you profile picture. Just take a picture of the named creature to get the special photo to unlock it, then quit the maze.
- Angel - Hell Frontier
- Skeleton Hound - Borderlands
- Vampire Hunter - Dracula’s Castle
- Gladiator - Ancient Arena
The Easter egg just gives you chat icons. To use them, on the world map: tap on the Gumball icon in the upper left corner of the screen -> then tap on the Gumball icon in the upper middle of the screen -> the icons from the easter egg are on one of the far right tabs.