Loony the Drifblim
Drifloon's evolution, Drifblim is used by one Gym Leader in the Pokemon games: Fantina, a ghost-type Gym Leader in Pokemon Diamond and Pokemon Pearl. Uncreatively named Loony, Fantina's Drifblim is the only Drifblim with individual significance that I know of.
In the anime and manga, Drifloon -- and later Drifblim -- is Fantina's main Pokemon. She uses it for transportation, companionship, and battling. She even styles herself in likeness of Drifloon and Drifblim, wearing a purple dress with a yellow X on it.
Ash battles Fantina's Drifblim twice in the anime, and it proves to be a formidable opponent, mainly due to its ability to shake off attacks. It defeats Ash's Buizel and Pikachu, only losing to Chimchar after Ash outsmarts Fantina.
In the manga, Fantina's Drifblim saved Diamond and Pearl from a deadly fall. It then carried the duo to Hearthome City, where it fought for Fantina in a gym battle.
Fantina's Drifblim is somewhat of a contradiction to the Drifblims and Drifloons found in the Pokemon games. It actually saved Diamond and Pearl's life, showing a good side to what's supposed to be a malevolent creature. I think like any creature, this probably has a lot to do with its upbringing, and is more a reflection of Fantina than Drifblim. Fantina hatched her Drifloon from an egg, so it has been with her since the beginning of its life.
Fantina's Drifblim has a lesser role in the games, not even appearing in her party during the gym battle in Pokemon Platinum. While she swapped Drifblim out for another Ghost-type Pokemon in Pokemon Platinum, I think Drifblim is still important to her. She still wears the dress she modeled after Drifblim, and she uses Drifblim in Super Contests, where it is revealed that she named her Drifblim Loony. (Super creative name, isn't it?) Loony is present in Fantina's party if the player chooses to battle Fantina again in a rematch.