Since at least one of my characters is a demon of sorts I thought it'd be best to look into monster concepts to both get an idea of what a good creature would look like. This also allows me to add much more variation to my research.
This piece done for an indy game by Deviant art user Endling features many variations and annotations but focuses solely on the concept that the monster is a large Lava based creature. These loads of variation and each would be acceptable as a final design, eventually it's clear Endling settled on the large horned reptile and it works really well. Clearly taking in the environment that is around the monster and applying those elements to the monster, this is presented by the rock like hide and vents for lava to flow from. The colour pallet is also taken from the volcanic area with dark reds and oranges being the main motif here.
What's a really good of this design is the fact you can only really see the head of creature leaving the rest of the monster entirely up to the player's imagination, this is both a great way to add interest to a design and also get away with not designing more than you need to. My personal favourite out of the bunch is the large insect type in the centre.
This next piece by Joy Ang, a conceptual artist for Bioware's Mass Effect and Dragon Age series, gives use a pretty scary looking demon. With large forearms and sharp features all over it's body the over all aesthetic tells you this is not a creature you want to run into. The design is almost iconic in how a demon should look and yet at the same time it's fresh and new due to little added details such as the way the jaw is almost unhinged and the texturing on the chest.
Again the main colour system shows up again with the main two being purple and black, with shades and variations being use but no colours that are beyond that spectrum has been used thus doesn't deter visually. Ang's art style also breathes fresh air into this otherwise standard design, with all the extra details the feeling of this design is something truly menacing.
This design for one of the summon creatures from Final Fantasy is interesting as the base shape for it is very similar to a medieval representation of the Devil as a Goat/Pan like creature with a main difference being that this creature has wings, the reason for the monster's appearance is due to it being based on the zodiac creature; the goat. Covered in a hard decorative armour for skin this design is almost a far cry from what you'd expect an ally to look like (or atleast from a western perspective where in the Devil-Goat is more well known).
The colours in used are predominately a kind of Jade and light/scarlet red, this is accompanied by a cream like colour being used on the tips of the wings. Overall the design is very lavish and helps translate the message that this is a creature of both great importance and power.
Already after looking into these three designs I have ideas forming in my head for Xangdra's demon form but I plan to look at atleast three more. When it comes to designing I will also use my Persona 4 and Monster Hunter art books for influence/inspiration as they're full of monster designs.