Brian

Brian (aka BBBBBD) was the second longest lasting Pokemon in AJ's party during Twitch Plays Pokemon: Crystal. Brian started off as a Pidgey that was caught near the beginning of the game. It evolved into a Pidgeotto at 3d 22h 53m and finally, a Pidgeot at 4d 7h 19m.

Name
Due to the success of Twitch Plays Pokemon: Red, many people were favourable to calling BBBBBD 'Bird Jesus' again, reasoning that it was his second coming. However, a more vocal majority wanted to move away from their previous game and wanted to call him something different. This led to a large number of name suggestions, as at the time there was nothing significant about him, especially as LazorGator was doing most of the battles.

There was a large number of fan-art displaying BBBBBD living under Bird Jesus' shadow, as many agreed that he had a large reputation to live up to as a Pidgey. However, this served to create the idea that BBBBBD simply wants to be left alone and be his own person whilst mistaken for the Messiah. This theme is similar to the movie 'Monty Python's Life of Brian', in which the titular character is mistaken for the Messiah. This is what led to people starting to call him 'Brian', which also fits with his original name that mainly consists of B.

Some people suggested that Brian should be called 'Richard Hawkins', due to his theorised pursuit to bring down the theology of Helixism after living in the shadow of its Messiah. This name received some backing but never stuck. Other people also began calling him 'Bird Brian', to bring further similarity to Bird-Jesus.

TPP Lore
Lore from Generation 1 was still fresh in everyone's minds at the time of Brian's catching. Many people began calling him Bird Jesus or 'Bird Jesus Jr.' As mentioned above, Brian became 'Just Brian'. In an interesting twist, Brian fainted in Mt Silver before the final battle with Red, and thus missed it. In some depictions, this truly made him just a normal bird.

While early interpretations sometimes placed him as related to Bird Jesus, it is now widely believed that he had no relation to the Generation 1 'Mon in TPP-canon. There are stories that do cross the two birds' paths before the events of Generation 2 however, with most depicting Bird Jesus as being amicable and encouraging to a younger Brian.

Brian's purpose in joining the party seems to be to forge his own path while finding his own identity. It is not entirely clear, but during the early days of the Johto Wars, zealots of the Church of Helix may have been acting in cruel ways towards 'Mon of the Pidgey line who did not conform to their ideal of Bird Jesus. This would have given Brian a very similar purpose for joining the team and becoming a Johto Heathen as Burrito.

After the events of Generation 2 and the final battle on Mt. Silver, some lore depicts AJ, Red, and their respective 'Mon joining together to journey to find Bill and make him answer for the losses to the PC. Other depictions have the team returning to Johto to live out their lives in peace.

Trivia

 * Brian shares his species with another Hall of Famer; in this case, Bird Jesus is also a Pidgeot.
 * Brian, along with Doctor Hoot, is the subject of the song "Doctor Hoot: The Curious Case Of Brian" by Church of the Helix Choir.