Ace

Ace, known as A ingame was a member of AJ's party during TPP Crystal for the Elite Four, but was deposited before the fight with Red. Ace was caught as a Raticate.

Origin
One popular theory on Ace's origins is that he was Joey's Rattata that was sent by the trainer to aid AJ on his quest, either evolving first or evolving along the way. People use this as an explanation for why Joey calls so often - to check up on his old Pokemon. This action appears in-character for Joey, who appears very friendly with AJ.

Other stories have Ace traveling with Solid Snake on Victory Road before AJ encountered them. This has led to popular art depictions of Ace travelling on Solid Snake's head.

Deposit
Sometime before AJ's final fight with Red but after AJ's initial fight with Red, the PC was accessed and Ace was deposited during Anarchy, leaving the party with five Pokemon. This was a combination of luck, and strategy on the part of some players who believed that Ace was holding them back due to poor statistics and level. Although there were initially efforts to retrieve Ace by those who liked him, the streamer removed Democracy mode from the stream later that day, leaving no safe opportunity to carry out an operation to do so.

From this event, some players interpret that the voices in AJ's head told him to release Ace, but he was able to fight back against them and merely deposit them. Some believe that AJ wanted to return for Ace once AJ was free of the voices and could navigate the PC safely. Other people see this action as benevolence on AJ's part. This is because Ace was indeed of a lower level, so AJ wanted to keep the Pokemon safe from harm on the rest of his journey to defeat Red.

Some like to believe that after being deposited, Ace was either sent back or allowed to go back to Youngster Joey, as a follow up on the lore that Ace was Joey's "top percentage" Rattata all along.

Name
At first, the Raticate was given the name A. Because he was such a blank slate and, at the time, had not done anything noteworthy, many people tried to come up with names. The most popular name was 'Ace', which fit with his given name and was slightly ironic given his under-appreciated nature.

Other popular names included 'Just A Raticate', which also led to 'Just A' and 'A Raticate'. Those who disliked him called A, 'The Rat', whilst others simply used his species name 'Raticate' as they were indifferent. All these names were deliberately uninspired due to A's perceived underwhelming nature.