I'm not entirely sure what filler in Aria would even consist of. If they added material not in the manga it wouldn't really matter since nothing happens 90% of the time in the manga anyway. I guess you're just asking how much the anime deviates from the manga but filler is just such a weird word to use in regards to Aria.
As for Ai, I believe she is an original anime character who later showed up in the manga. In the manga Akari writes her letter to an anonymous person.