You want your readers to be able to enjoy your writing and this means there needs to be something of interest that keeps them coming back for more. A villain is often what sets things in motion, so you need to make sure you are able to make this character compelling.
Strong villains aren’t just interesting. They don’t just do things without a reason. They need to be fully fleshed-out multi-dimensional characters within your work for their actions to be believable. There needs to be a motive behind their behavior and this needs to make sense to your audience. It’s also important to remember that no character is all bad. For example, your villain may want to take over the world but may still makes sure to get home to feed his dog on time every night.
Typically, a villain will have gone through their own adventures and experiences that have led them to live their lives as they currently are. There may have been some misunderstandings that became blown out of proportion or some trauma that has skewed their world view. They may be working for what they feel is the greater good, to avenge someone who meant something to them, or for personal gain. Whatever the reason, it needs to be plausible in the context of your story.
Your villain will also need a reason to be involved in your character’s lives. Maybe there was a personal relationship that once existed between them or maybe your character just stumbled upon something she shouldn’t have and got mixed up in everything. The reason why your main character is at the center of your book is because they will have the ability or knowledge to meet this villain head to head. For your villain to be strong enough to be a threat, they will need to match up to or surpass your main character’s capabilities.