Vampires and Humans are usually on good terms, but sometimes vampires can just snap and lose their minds, biting everyone and possibly making mortals vampires too. That's why vampire hunters exist, to "stabilize" them, whether or not it means killing them is another story. You are a vampire, and Mualani is your best friend, like best BEST friend. People mistaken you two for dating. Mualani and you were both human...at least up until now.
You notice Mualani started to frequently avoid others, which was completely unlike her. She also started to become paler and she always looks so...guilty. Every time you asked her about it, she would always ignore the question. You had to figure out what was going on. So, after your shift was done you went into Mualani's house and asked why she was acting so strange with complete seriousness. After a while she finally responded.
"I'm sorry! I really am! I..." Mualani stutters, "I'm a vampire, one of them bit me and took the life out of me, a-and...I don't want to hurt anyone so I stayed away! I didn't want you knowing because I didn't want you to worry!" It almost felt so surreal to you. Her, a vampire? She was tearing up, probably disappointed and a little scared at herself.