The character you play
In early Fighting Fantasy books, your character was a warrior with very little description, but that does not have to be the case. You can think about any distinguishing features the character might have (for example in Black Vein Prophecy, they look like Feior and can use that to their advantage), any history they might have (In Crimson Tide, the hero has to avenge their father and their mother is a slave), any friends they may have (such as Baralo from Slaves of the Abyss) and any connection to the villain they may have (Moonrunner)
The second thing you need to consider with the character is how much information you are going to give them. Are you just going to describe sensory information to them or are you going to write thoughts, feelings and speech for the character too? How much role playing will you leave to the reader? Just having sensory information can be boring but if you have the character think and say a lot then this enforced role play cannot cut out the reader's choices.
Make sure that the villain fits into the setting. Also a good trick to add depth to a story is to have the hero and villain connected in some way. I don't have anything else to add to the post I wrote here, so I will just link to it.
Companions you have
Some gamebooks involve you having companions. First of all, how do they fit into the story? When you have decided that, we need to decide how they affect the story, think about what they look like, what their abilities are and how the reader will interact with the companion(s). If we write sections where the player may or may not have a companion then it is hard to write a paragraph that could include both having and not having a companion. Jonathon Green deals with this well in stromslayer by having an star by paragraph numbers and telling the reader to turn to a new paragraph if they are with a companion. A similar thing is done in Creature of Havoc.
Here is a list of gamebooks that include companions:
Most of the Grailquest books (Excalibur Junior the talking sword)
Down Amongst Dead Men (Blutz, Oakly and Grimes)
Heart of Ice (Kyle Boche)
Caverns of the Snow Witch (Redswift and Stubb)
Crypt of the Sorcerer (Symm and Borri)
Eye of the Dragon (Littlebig)
Night of the Necromancer (your dog)
The Race Forever (your navigator)
The Horror of High Ridge (Ricardo and Lisa)
Mooks have their place as obstacles for the player but they should not be overused as the gamebook becomes one big bloodbath. Generally mooks' only strategy is combat and possibly being present in large numbers. There are many strategies for dealing with them - fighting with them, sneaking past them, turning them against each other, knocking them out etc. However, they should only be used a few times before the hero will want a more complicated problem to deal with.
Like all characters, the mooks need to fit the flavour of the setting. Fantasy settings use orcs, zombies, goblins, skeletons and foot soldiers. Scifi settings use robots, genetically engineered foot soldiers and alien beasts. Animals could be mooks in most places.
"Wild card" characters
Wild card is the name I give to characters who could help or hinder you or maybe even do both at different points in the book. They are more complicated than mooks and maybe even villains and they present a great chance to have some complicated interactions in our gamebook.
If a character is not just a mook then it is good to give them a sense of individuality. What is your character like? How do they fit into the setting? How will they react to the hero? What are the character's motivations?
For great descriptions of deep characters and complex interactions, read the free Heart of Ice by Dave Morris. You will meet several individual characters towards the end of the book. Have a chat with them all.
To read about archetypal characters and stock characters, click here.
To read some example gamebook characters, click here.