Shin Megami Tensei - Demikids has your starter Mon. The very first monster you get, and your partner for the entire game, only gains levels by absorbing other monster's exp through fusion. Taking into account that you'll likely need most of your other monsters for fusion, you'll usually end up just mass fusing dozens of cheap monsters to get your starter to level up. To put it differently: This is like getting a Pikachu that has to devour its opponent every time it wins a battle. By the end of the game, your starter is essentially a walking mass of hundreds of souls.
Plenty in the demon's designs. The monster design brings forth references to the mythological being it's based on, to other games in the series and sometimes to pop culture as well.
Once you get over Mara's face, you should notice it riding golden chariot with many sharp blades. Then you realise it's embodiment of our dark desire - lust, greed and violence.
At first, it's weird seeing how Angel - a Divine being - is dressed in the series. It makes sense once you notice that she also wears a leash. After all, they are guardian angels that are bound to a human being on birth.
Melchizedek. By far one of the most outlandish angel designs, but makes a whole lotta sense when you think of it.
First off, he looks and acts more like some kind of Super Sentai hero than an angel. Well, he's the King of Justice. Just try saying that out loud.
Second, he's faceless. In the Bible, Melchizedek is described as "without father, without mother and without genealogy".