Creating believable, interesting characters is one of the cornorstones of writing any fiction, from a short story through to a series. Like all writing skills, the ability to do so is as learned as it is in -ate, and practice always helps.
(1) Picking a sex and a name are obvious starting points. Names will be influenced by other major characteristics, such as nationality and job. For example double bar names are more common among professional types such as lawyers and academics than they are in the service industry. Artists often have quirky names, by both accident and design.
(2) Give them an age and root them chronologically. After you have determined their date of birth you can create a rough timeline of their lives including all major events such as starting schools, starting collage work, all their firsts that they have experienced from alcohol to sex to violent encounters. When they married, if they did, or moved in with a partner or their current flatmate.
(3) Give them a look. Start with the basics height, weight, nationality, skin colour, eye colour, hair colour, hair length. Give them scars and tattoos and physical quirks, and dress them. Are they attractive? Or ugly as sin? How confident are they? Are they naturally beautiful or do they try to hard? Are they conscious of their appearance or indifferent. Their income will shape some of their style choices, but style itself does not belong to any class.
(4) Give them a family tree that traces their parents and siblings and off-springs and other relatives that are significant to the plot.
(5) Give them a personality. Start out in broad strokes and then refine. Are they an optimist or a pessimist? Are they mild mannered and pleasant? Or quite rude and abrupt. Are they alpha or beta? Are they good or bad? Once you have determined the bigger traits you can start to add layers. What about contradictions? They may be notoriously mean, never buying their round in the pub, but have one person or cause that they lavish their money on?
(6) Give them a present. What do they do? What do they earn? Where do they live? Do they have financial security? Do they have emotional security? Are they happy with their current lot.
(7) Give them a past. Once you know where they are at the start of their story, you can figure out better how they got there. A doctor, for example will have years of study in her background, if she’s young, she may have huge debt, if she’s older, she might have accumulated wealth. A successful actor may have spent years moonlighting as a waiter. People often tend to do what their parents do, or reject their background entirely.
(8) Make a list of their likes and dislikes. What their favourite food, film, book? What’s there guilty pleasure. What do they do in their free time? What are they passionate about, bearing in mind even the apathetic and cynical, have causes and people that matter to them? Who do they admire? Who do they hate? Who do they envy and why? Are they authentic, or a hypocrite, or like many of us, a bit of both.
(9) Dig into their secrets. What do they feel guilty about? What are they ashamed of? What do they hope no-one ever finds out about them? Who have they lied to? Who have they lied for? Who do they trust? Who do they fear. Broadly and more specifically. For example a woman who has been subjected to a sexual assault may fear all men, but especially her attacker.
(10) What is their relationship to modern technology? If a piece is written in contemporary times, the characters must interact with modern communication systems such as the internet and all its corresponding social media, in age appropriate ways and devises. For example the young ones all have phones and communicate through them almost all the time, including to each other, whilst sharing physical space. Many of your older characters will also engage with social media for work and pleasure. At least some of your characters will access the dark web.
All of the above are just suggestions and should only be used if they prove useful. There is no one way to write a character and the most carefully crafted protagonist can find themselves acting in an unexpected fashion as planning meets writing. The important thing to remember is to enjoy creating characters and the freedom knowing them better brings to your story.