I am currently forming a relationship with a person diagnosed with BPD. Here are a few of my thoughts.
Call them co-dependent. Normal relationships are give/take, give/take. People who don't take, don't give any satisfaction when a person wants to give.
I believe this is a legitimate concern, but a stereotype that need not always be true. I wanted to be careful mine does not fall into overly unhealthy patterns, largely out of respect for my partner. I think we're both committed to it being as healthy as we can make it. In my situation, I know that it does have a strong mutual component, with give & take in both directions, as any healthy relationship must. My partner has helped me in very deep and profound ways, and I've accepted what they've given me with gratitude and not infrequent marvel at their wisdom and insight. I've trusted them with my own pain, bared my soul to them, received their support and caring, and tried to change myself for the better. Support and caring goes both ways in roughly equal amounts. Is that a codependency? I don't believe so.
Just because things
can fall into unhealthy patterns doesn't mean they
must. I think if one is willing to poke one's head up from time to time and sort of steer away from the biggest potholes, a BPD/non relationship can be relatively healthy. It doesn't mean it will be a perfect thing, or easy, or always smooth sailing, but nothing ever is. Non/Non relationships are not perfect either, and they also have ups and downs. Humans are human: imperfect, with our own needs and wants and fears. In the end all we can do is our best. There is strength in accepting imperfection.
In the beginning, I was frightened of engaging on a deep and emotional level with a person with this diagnosis. It helped to be honest with my partner about it: "Look, I'm scared..." Now, I'm not afraid any more, largely due to good communication with and help from them, a willingness of both of us to be open, and being humbled by the amazing strength they posses in the face of something that can be frightening and isolating.
So... the effects? At least for me, there has been learning, true mutual support, friendship, profound conversation, and a better acceptance of myself and my own flaws. I cannot know the future, whether it will work out, but I hope it will, and I do my best. I owe both myself and my partner no less.