The reason she became a team member is because she wasn't affected by attacks 12 times in episode 7.
She is Chugga's first Ghost Type.
Chugga had a prediction that the stupid A.I. would be a laughing stock with Affection being unaffected by Normal, Ground and Fighting-type moves, which the AI use quite often on her.
Chugga used a counter that shows each time Affection is unharmed from attack. It continued throughout the Let's Play with the end counter being 31 (not counting allies using earthquake and explosion).
Although he thought Hyper Mode would be useful, Affection ended up disobeying him and almost KOing Saikou.
For a few episodes, Affection would constantly get into Hyper Mode. This is most prevalent in episode 11.
When she was purified, she got over 21,000 exp. points, and got a level quite higher than the rest of the team.
In episode 11, Chugga decided to keep Misdreavus in Hyper Mode for the fun of it. A few minutes later, Chugga found a Protein and couldn't give it to her due to her being in Hyper Mode.
Chugga mentioned her a lot in his Pokémon XD walkthrough, mostly when facing another Misdreavus.
Her original name might have been "Effection," after all the times that the AI used moves that did not affect her.
This is somewhat debatable, as Chugga mentioned he perceived the commentor to have spelled the name incorrectly.
Affection was the only Pokémon in Chugga's Pokémon Colosseum LP that got an evolution in a later game that was not mentioned in his bio of the species.
Affection is Chugga’s only main team member, so far, that isn’t not at its final evolution. This is because Mismagius was not introduced until Pokémon Diamond, Pearl, and Platinum. Murkrow, another Shadow Pokemon in Colosseum, also got an evolution (by the same cause as Misdreavus' own, no less) in Gen. IV.