Hi AnonChick,
You are welcome here. It's a good place with many people who will listen, share their own experiences, and what they've learned about themselves. You will find support and surely some answers. Don't worry about getting the terminology right. It takes a while to learn all the jargon, and it's only useful as a kind of shorthand.
Did you have a look at some of the resources here? This might be a good place to start:
https://bpdfamily.com/content/what-borderline-personality-disorderI am not sure exactly what is happening in your situation. Do you mean that you think, or know, that your partner created some kind of profile that he is using to write to you undercover? To pretend that he is someone else and so carry on a conversation of some kind with you?
I don't think that specifically is the kind of thing that people with BPD do. It is certainly slightly odd behaviour.
If I've got that right, why are you sure that it is your partner and why do you think he might do such a thing?
My immediate idea is that it might be some kind of testing behaviour, to see how you will react. My speculation about that might be that it's the kind of thing a jealous and insecure person might come up with. There may be other possibilities.
Can you tell us more about your relationship and yourself? How long have you been together? Do you live together? What is the relationship like in general?