Promise me... Promise me you will banish the darkness, and restore peace to the world...
Aria Benett is the last of the four Maidens of Water, a clan of priestesses linked with the Water Crystal. When the Warriors of Light discover her, she is at death's door in a sunken boat, with only an old man who happened to find her looking after her. The party gives Aria a potion, prompting her to get back to her feet and work with them to restore the world from its flooded state. She helps them to get a small fragment of the water crystal that still retains some light and accompanies them to the Cave of Tides to resurrect the crystal. However, the Warriors of Light are attacked by one of Xande's minions: Kraken. Aria takes an arrow that was aimed at the Warriors, but between that and her already less-than-stellar health, she only has time to bless the Warriors before dying. The collapse of the Cave of Tides shortly after means that the Warriors don't get much time to react before they themselves are knocked unconcious for three days.
Characters dying for the main party is not a new concept for Final Fantasy, as seen in II just a game prior. However, Aria stands out due to the tone shift of the entire flooded world section. Final Fantasy III keeps a light tone for most of the game: The guests up to this point have been largely playful in one way or another, only one scenario so far has an on-screen death (the poor dude who explains the situation in Tokkul right before Castle Hein) and the all the way through, the world map plays the upbeat "Eternal Wind".
Then you leave the Floating Continent to find a massive ocean with scant few landmarks (Hope you brought some gnomish bread!). The music changes to the melancholic "Boundless Ocean". And then you meet Aria: The first character in the series with a character-specific leitmotif, itself a beautiful waltz. Combined with her death, Aria's section of the game is probably one of the most memorable parts of the game, at least plotwise. Which makes it jarring when the game goes back to its (comparatively speaking) light-hearted antics and Aria is never mentioned again.
The remake builds on Aria in a few ways. The dialogue is overhauled to make her a bit more focused on her duty and the game puts Luneth more in the center, mirroring how each of the main characters connects to one of the guests. Still, her role is very close to the original and this pales in comparison to how the unused text builds on her...
Additionally, Aria has the honor of being the only character other than the four protagonists to appear in the CG cutscene, praying within the wreckage of the ship. Clearly the devs agreed in Aria's scenario being one of the game's most memorable moments.
During battle, Aria uses White Magic, much like Sara before and Alus and Unei after. Aria does not have a physical attack.
Quite a bit actually. Aria's interactions with the Warriors of Light more heavily showcase her duty-focused personality and her interactions with Luneth. The latter is especially interesting due to how Luneth seems to change around her; He's still excitable, but Luneth shows far more concern to Aria than he has up to this point. Still, Aria reassures Luneth despite his concerns, causing Refia to say that Aria is tougher than all of them. Still, the scenes do show an interesting contrast between the two.
Aria has other scenes as well. She goes a bit further into how the water maidens handeled the crystal (as well as mentioning that there used to be three others who "ran out of steam") and she muses that the Floating Continent did not exist before the darkness appeared (foreshadowing the twist regarding the continent's existance in the remake).
Notably, the unused text reveals that the abruptness at which the game moves on from her death would have been avoided to an extent: Luneth wakes up overhearing Ingus and the old man discussing it, it's implied that Ingus' frustration with Luneth and the others is due to them being not so phased by Aria's sacrifice and Aria is mentioned on two occasions later (one of those alongside Desch, whom the party believes is dead by that point).