For this part it's important you already have a character you want to design a cast around, as stated in the requirements earlier. I will use one of my own characters as an example.
It's really beneficial for cast cohesion if you have a clearly defined theme/framing device. For example, my cast is going to be based on Greek mythology. This makes it easier for me to make character design decisions, since I already have a popular subject to base them off of, and people will automatically associate my characters with each other as long as it's apparent that they're based on Greek mythology.
We'll start by analyzing every part of his design, the inspiration behind it, the style, the shape language used, his personality etc. This is going to help us define what makes this character unique, but also what we can use for cohesion in the rest of our cast.
Name: Eros.
God of love and sex, son of Aphrodite and Ares.
Instead of using a bow and arrow, he punches you in the stomach.
Uses pink magic.
Pansexual.
A little arrogant, pretty intrusive, charismatic.
Needs to look sexy.
Inspirations:
https://i.pinimg.com/564x/ca/79/46/ca7946615070395613c9fec29f85ec76.jpg
Y2K fashion trends.
Greek god family tree.
Yap1co's art.
Theming:
Greek Mythology.
Modernisation
Popular iconography (hearts and symbols like that)
Color scheme:
Pink, red, black.
3 colors excluding skin and eyes and such.
One really bright accent color.
Shape language:
Sharp edges, even on more rounded shapes.
Clean and readable.
Exaggerate proportions.
Design patterns:
Character motif (heart) plastered all over.
Non literal interpreatation of tools or weapons (bow and arrow is an arrow tattoo, wings are a tattoo)
Style notes:
Thinner lines, can be messy.
No shading, except for black shading where necessary for definition.
Chunky muscles.
Cleary defined areas of detail (left arm, face, pants) 2/3.
Big eyes and feet.
Flat color, only use highlights on stuff like pleather or latex, use a glow effect on magic.
These are the main things I thought about when designing him. Some of these points are exclusive to Eros, but many of them can be used for the general style of all the characters in the cast. It's important to pick the points that are specific enough to form a cohesive style, but allow for enough diversity so not all your characters look the same.
In my cohesion guide for "Pantheon" I've placed the most important rules for cohesion on the main page, so I can always see them when Im designing.
Here is a link to the board: https://app.milanote.com/1OLr6s1JdWh8aa?p=SA6pUIkB4Vg
You'll notice I've picked points that are not unique to Eros alone, but can be applied to a whole cast of characters. I've decided on clear limits, like 2/3 main colors per character, which will keep me from going overboard, but still allow me enough freedom for a lot of variation in my cast.
These are only the most obvious and important points to stick to, more detailed and nuanced rules can be found in the Visual and Narrative columns on the board.
The devil is in the details, which is why filling your Visual and Narrative column with nuanced information is so important for a good cohesion guide. If you work alone you can maybe get away with shoddy moodboards, but since this cohesion guide should work in a team setting, thats a no-go.
Moodboards:
I'm not gonna tell you how to make a moodboard, but I do advise making different types of moodboards for different aspects of your character designs:
General inspiration (this should portray the general vibe of your cast/concept).
Fashion inspiration (clothing styles and clothes from the period it's based on).
Artstyle inspiration (references to different artists and styles).
Individual character moodbards (if you want to go deep).
Linkt to moodboards: https://app.milanote.com/1OLrng1WAEZv6K?p=SA6pUIkB4Vg
It is a little more work than just throwing everything you find into a big messy heap, but this method makes it way more manageable and uncluttered. In my experience it also motivates me to go deeper for every moodboard and look for more unique references.
Style guide:
Most of the information about the style guide is on the previous page, and is pretty self explanatory. I just translated the notes I took when I analyzed the design into more general points for the style. It might change a little bit when you design more characters and find new things to add or old things to edit, but make sure you keep the core of your style guide intact.
Link to my style guide: https://app.milanote.com/1OLrnz1WAEZv6L?p=SA6pUIkB4Vg
It's important for cohesion to have a solid narrative context to design your cast for. I decided on Greek Mythology to frame my cast, because it has a lot of iconic imagery, characters and stories to take inspiration from.
In the thematic inspiration board, I'm collecting different videos, sites and documents that I can refer to when I need to get inspired. I made a list of greek gods and how they relate to one another, so I can see what the dynamics might be between the cast members.
I included audiobooks about greek mythology and heroes by Stephen Fry, as he makes the stories very digestible and easy to remember, keeping the most important details in and giving an accurate description of each characters' personality. This makes them feel very modern and relevant, which in turn is the basis for my entire concept.
I also made sure to have the most important and obvious themes be in the column on the main board, so they're always accessible and visible.
The extra column exists to file away things that don't warrant an entire column or board of their own. Things like playlists that inspire you while drawing, or videos that have an energy that fits well with your cast design.
I also put in the wikipedia page to the game "Hades", since it deals with pretty similar concepts.
This section is pretty empty for now, but may expand later on.
Now that you've made a styleguide, made moodboards and categorised your narrative inspirations, it's time to apply them to a new character. On the next page we'll be expanding our cast with the information we've documented in the cohesion guide so far, but we'll also be adding to it.