Designing a character

Disclaimer: All this is based on my own experience in writing.

To write any story, you will need to design some characters. For some, you may get away with under ten characters for the whole novel, but some hit the hundreds pretty quickly (*cough* fantasy *cough*).
There are many things that you have to decided about a character when designing them. I personally store all the information in a database for reference (due to the aforementioned problem of characters going into the hundreds). So, without further ado, I will get to some of the questions you have to ask and the things you have to decide.

Basic: 

Basic information about the character.

1. Full name: This may sound stupid, but knowing your character's full name will also tell you a lot about them. How many given names do they have? Which one do they use? What about surnames?
2. Preferred form of address: first name, middle name, nickname, or the surname? Or do they prefer to go by full-name basis? What about titles? Do they insist on it?
3. Age and/or DoB: Date of Birth is important only if you have the dates fixed.
4. General appearance: You may think you can get away with not defining it in a book but you'll be dead wrong (at least in most cases).
5. Right of left handed: This is important especially if a lot of action is involved, or if it fits into crime or a related genre. It'll determine how you reach for something, how you wear a bag or something of the sort, how you instinctively react to an attack, and so much more. By the way, very few people are truly ambidextrous - they may be capable of writing comfortably with both hands but may instinctively reach for something with their left hand, for example. If your characters are non-human, you can go ahead and ignore this.
6. Brief character history: It doesn't have to be long, but their general backstory of a character can shape their behavior. The bearing it has on the story depends on the type of story, of course.

Personality:

1. General temperament: Describing it may be tough, and the character may evolve away from your initial definition as you write, but putting in the effort to define it might be worth it.
2. Character quirks: Not everyone has them, but it could e a useful way to distinguish a character.
3. Communication style: Again, this is difficult to define but can go a long way in developing a character's identity.
4. Likes and dislikes: This can define a character quite well, too. For example, if one defines their dislikes as 'messy rooms' and another says 'chaos', it's quite reasonable to assume their personalities aren't too similar.
5. Dress style: Again, this can tell you a lot about them. Do they stick to a single colour? Do they prefer one particular type of fashion? Is there some article of clothing they always have (say, a scarf)?
6. Relationship with fellow sentient creatures: Some people can make friends easily, some can't. Some aren't particularly invested in their relationships with people, some are. Some only genuinely care for a small group - be it their family or their closest friends. Others will go out of the way to help anyone. Where does your character fall?
7. Relationship with machines and non sentiments: This depends on your work, but can be very important.
8. Level of intelligence: This is hard to quantify, but noting how well a character does on various criteria that measure intelligence may help.

Motivations:

1. Goals: What is your character's goal in life? In real life, not everyone has a defined goal, all your characters may not need one. How important are these goals?
2. Fears: What do they fear? Is it failure? Or the loss of something? Or is it death? Or do they claim that they fear nothing?
3. Principles: What code do they live by? Are there any principles they're unwilling to compromise?
4. Attachments: What are they absolutely unwilling to give up?
5. Approach to achieving their goals: How much effort are they willing to put into it? How is this energy directed?

Further information:

1. Phobias: Again, not everyone has those, but depending on your work, it may be useful.
2. Level of religiousness / nationalism: If you aren't willing to touch these very messy topics, it should be fine (depends on the type of story). However, this can add another facet to your character. There are various levels of religiousness, too. Some may follow the religion to the letter, but to others it's just a part of their identity, and you find people everywhere in between these two extremes. The same scale applies to nationalism.On the other end, you will find who are ambivalent to the whole concept, and others who will reject it completely.
3. Disabilities: If the entire character is built entirely around some sort of disability, mental or physical, then it is a caricature, not a character. That being said, having a disability can add a lot of depth to a character. Also note that not all disabilities, especially mental ones, are obvious at a glance (physical ones aren't exempt either - colour blindness for example).
4. Allergies: Quite specific, true, but a lot of people have them, and they can complicate things considerably. It can apply to anything organic (or machines too - like the oddly specific inputs that make them go haywire).
5. Language fluency: This may also be oddly specific, but since it determines who your character can communicate with, and how well that communication will go, so it's pretty important. Also, it's entirely possible for someone to understand a language but have trouble speaking it, or have trouble reading/ writing it.
6. Familiarity with technology: Again, possibly oddly specific, but it will determine how much your character can gather, how well they can communicate, and how fast they'll adapt to any technology that becomes available. This doesn't refer to just computers - being able to forge a sword is also being familiar with technology.
7. Skill levels: What sort of skills do they have, and how proficient are they at it? The specific skills in question depend on the story/setting. Being able to use a long sword will be useful in a medieval setting, but if your story involves a series of battles fought out of visual range or with robots - you get the idea. Incidentally, that example works the other way too.

A final note: Defining all this is a lot of work, but for me, personally, it always pays in the end. Details have a way of coming out in the narrative, and a little off-hand reference to some noodle incident, or a slight dissonant reaction can add a lot to a character.
To conclude, it's not necessary to define each and every aspect stated here for every character, and the attributes listed here may not fit every type of work. This is not an exhaustive list, either - depending on your work, you may need to consider many more criteria. The purpose of this article is to help you organize your thoughts, and hopefully, take a more organized approach to character creation.

Happy writing!