The one person from your past that you can't get over, but can never obtain.
Similar to the "Lobster" or the "Boo", the "Unreachable" is the person that you've liked/loved in the past, had a strong connection with, and always end up thinking about every now and then; the only difference is that you never really had them to begin with.
Lisa: I miss Brian...He's my unreachable.
Phyllis: You mean your lobster?
Lisa: No, hoe, my unreachable....