Am I more than friends with my ex's best friend?

You both need to change it form an "unsaid thing" to a said thing. You need to tell him how you really feel about this relationship and seriously ask him what he thinks the future holds in store for the 2 of you. Number one topic you BOTH need to discuss is your ex. If your lover is/has been REALLY good friends with your ex, he will have to man up and tell (himself and alone) your ex if you BOTH want to continue the relationship. If you BOTH do not see a future together, then my advice would be to tell your ex what has/is happening before he catches you two in the act. If he catches you together, it will be a very, very ugly scene. If you discuss this with your ex, it may very well be a very ugly scene, but will still need to come out in the open, and should eventually blow over. The friendship your lover has with your ex will definitely be over, no if's and's or but's. Is your lover willing to forsake that? Are you willing to deal with never having contact with your ex again? Once he finds out (whether from "busting" you together or by you two telling him), he is likely to break off contact with you both or worse. Are you BOTH willing to do that? The second topic to discuss is your lover's passive refusal to consider you/introduce you as his girl/girlfriend/etc. It basically cannot be a secret any longer. If you BOTH decide to continue on together, you need to tell everyone. Secrets are only fun until someone gets hurt. Everyone is out for pain until the truth is out. Good luck.