We have a stalker

Some words on paper made Frank and me embrace the wall. I wasn’t literally hugging it like that coward, but something was tightening in my chest, crying for help.  
That horrid message: 
Greetings, Asshole! 
You must think you are so smart for attempting to make a fortune by selling secrets in a billionaire’s estate. After keeping a close eye on you for so long, your treasure chest is finally in my possession.  
Do you want to know what I wish to do with it? Let’s see. I may use it to show your greed to Mary Sinclair. My proof does not end with the envelope and pictures of you entering Mr. Sinclair's office and placing it there. 
Come to the library in seven days if you don’t want that to happen. I love the thrill of blackmailing. Let’s buy your freedom. 
Yours hatefully, 
A person who is invisible to you yet watches your every move. 
We were clinging onto opposite walls, and the letter lay in the middle. It was a cursed object. Someone was onto us. The hunters had become the hunted. It would have been better if Mr. Sinclair had found the envelope. His blameless wife Mary would kill us for ratting her out for money. 
“Burn it, Amelia,” Frank cried. He frantically jumped and hugged the wall again. “I can’t watch this. I can’t watch someone steal the opportunity I have waited for so long. Burn it, or me.” 
I carefully moved closer to the letter and hesitatingly lifted an edge. Upon realizing that the blackmailer couldn't jump out of paper, I took it in my hands and reread it. 
There was one thing I noticed after going through it again. The anonymous blackmailer addressed one person, and they claimed to have been keeping an eye on that person for months. It clearly wasn’t me.  
“Frank, first, you need to calm down and listen to me,” I said, slowly approaching him with the letter.  
“No. I don’t want to hear it.” Frank ran to another corner, crying and screaming. He was acting like a demon getting chased by a cross. “Somebody knows what we have done. They hate us with a passion and want the worst for us. I don’t think my soft heart can handle so much loathing.”  
Angrily, I crushed the paper between my fists. That melodramatic jerk was pissing me off.  
“Frank,” I said, shoving the paper into his face. “Look at it. Somebody only hates you, not me. They are talking about one asshole and have been following that piece of shit for a long time.” 
Frank snapped out of his whiny and screaming little girl character and returned to his mature self. “Give me that.” He snatched it from my hands, and his eyes speedily confirmed it. “Oh my God.” He covered his gaping mouth. “This person is only after me.” I nodded. It was too obvious. “It’s a typical case of infatuation,” he continued. 
“What?”  
“It’s so obvious, Amelia. A young boy or girl gets a new job, starts liking the most handsome coworker, and tries to get his attention but is too scared because they know he is out of their league. The person then goes into depression and tries to stay away. However, they keep on getting attracted to the charming stud. They don’t realize when their little crush develops into a sinister obsession, and soon they drop to the level of stalking and blackmail.” Frank finished his story with cinematic applause. There was my confirmation that my new friend lived in a delusional world and probably heard voices in his head telling him all these lies. 
“This person has called you an asshole, Frank. Are we even reading the same note? It’s a hate note, not a love letter. Get your brain examined for injuries, dude,” I gave him a reality check.  
“I can read perfectly fine. That is why I concluded it to be a sinister obsession. You’re so dull. You must understand what’s not written here. For what other reason would a person hate me and want revenge?” said Frank. 
“I don’t know because you were not nice to them or did something unforgivable. Sorry to burst your bubble, but you don’t seem the nicest person. I have only known you for a few days and can tell you are capable of very dark stuff,” I replied.  
“I could say the same about you.” Frank walked away from me with a pouty face. “You’re mean. I can’t believe you would call me the problem instead of helping me to figure out who this person is. If I go down, I am taking you with me.” 
“Sorry.” I joined my hands apologetically. “I didn’t mean to hurt your feelings. All I am saying is that you should consider all possibilities. Maybe this person hates you because of something you did, sheer jealousy, or an unhealthy obsession like you did. We can’t say until we find him or her.” 
Frank nodded in agreement. “You’re right. I overreacted there a bit.” 
“It’s alright. Now, think very carefully, Frank. Who hates or loves you? Are you suspicious of anyone? Is there someone that lurks in your shadow without a reason? Think.” I urged.  
“Hm.” Frank squinted his eyes while staring into space. “Many employees are creepy here. I suspect some of them to be involved in questionable activities. But who could love/hate me with a passion for no apparent reason.” He thought for a while until his jaw fell open suddenly. “There is one woman, Amelia. She is too shy and always laughs nervously around me. I have noticed her staring at me from afar, and she gives me very unusual compliments and loves to ask me stupid questions. It almost feels like she is suffering from amnesia or finding excuses to talk to me. It must be her. Oh no, it is her. She knows I am gay, and that is why she wants to destroy my life. If she can’t have me, no one can.” 
Frank was again acting delusional, but at least it was a start. “Take me to her and allow me to observe how she treats you before considering her a suspect,” I said.