I would say more of a guest star. Someone could read the story with practically no knowledge of real life Adam at all and still follow it; he would read as an original character, and I don't think that it would negatively impact someone's enjoyment of the story. Obviously if you DO know of him it may heighten your enjoyment, but I don't think it's necessary at all.

He would be romantically involved with Neal within the story, but the main pairing by the end of the story is Neal/Peter/Elizabeth, with Adam's participation being more of a one-off or catalyst for the OT3 to happen.