Every character in a dream is you. Who else would it be? That means there is a part of your brain capable of manufacturing personalities separate from your own ego identity. It is entirely possible to manifest these identities while you are awake, usually through advanced shamanic techniques. These are known as tulpas, and they can be quite dangerous. Tulpas begin as very simple archetypes, but the longer they exist, the more experiences and personality they acquire. Over time it’s possible for the tulpa to become more powerful than your own ego-self and to supplant it. This is generally referred to as “demon possession.”
My suggestion to you is to use some simple purification and protection rituals. It’s not that the rituals themselves have some kind of intrinsic power, but that by performing them you empower yourself through suggestion. There are plenty of purification rituals available online in a variety of mystic and shamanic traditions, but salt is the most universal and common. Sprinkle salt in the corner of each room you want protected, then sweep it up the next day and throw it away. Replace the salt each day.
There are more advanced energy-manipulation techniques (such as reiki or tantra) for controlling and/or banishing tulpas if the salt purification doesn’t work, and even simple visualization and Jungian active imagination exercises can be useful for doing tulpa work, but it probably won’t be necessary. If you’ve accidentally created a tulpa, then you probably already have the intuitive knowledge to dispel it.