Mirror is an intertype relation of intellectual stimulation and mutual correction. The pair shares common interests, but differ slightly in thought process and methodology.
Initially, Mirrors find plenty of things to talk about. They easily understand each other's philosophies (and usually find that they agree) on basically all issues. They are surprised to find that the other can provide a subtly different outlook and recognize valuable things they themselves would tend to overlook. As the relationship gets closer, their differences become more of an obstacle. When everything is apparently all said and done, one Mirror will point out something they consider to be a loose end. This perplexes the other, because the point appears to them inconsequential and a distraction from the main point of the process. They may even think the other is intentionally derailing their efforts, because of their otherwise understanding attitude. This perpetual sense of almost understanding leads to some frustration between mirrors, despite it being a generally positive relationship. Any tension is easily resolved by creating some distance; confrontation is almost never necessary, and if it happens is quickly forgotten.
Perhaps more than any other relation, Mirrors can stimulate each other's creativity and work in tandem on the same project, but this interaction is primarily intellectual (i.e. work-related) and does not result in a feeling of closeness or needing the other on a more instinctive level. While they may find the discussion interesting at first, too much of it can lead Mirrors to have a sense of emptiness and disappointment about the relationship. They complement each other within their shared strengths, leaving an entire half of the informational world essentially neglected. They reinforce each other's attempts to engage the Super-id, but these usually remain half-hearted, continually returning to the comfort zone of the Ego.
The intellectual stimulation and surprise involved in Mirror relations make it quite durable and friendly even over a long period of time. Thus, Mirror is a common relationship among friends.