When I first came up to visit my boyfriend, I had the same worry, and I asked him to write the letter as a friend. As I later found out, this wasn't the wisest move - Border Agents are trained to look for giveaways or inconsistencies, and they quickly found out the actual nature of our relationship. After "misrepresenting information", it was only through a very kind guard who allowed me to explain myself that I was allowed in at all!
I'd recommend to just be up front and honest. As was explained to me, Border Services aren't trying to keep couples apart. Border Services and Immigration are concerned mainly with verifying that you'll leave Canada at the end of your visit. The fact that you're visiting a significant other will raise a few flags, and you'll likely have to answer a few more questions, but it's much better than appearing as though you're trying to hide the relationship.
Can a work permit holder invite a friend or boyfriend here? Would that be possible? or you should be a permanent resident or an open work permit holder?