honestly I can't give you a proper answer, many ppl decide to either i6 lynn (easier theatre clear) or i6 nifle (only if you have embla inflorescence - bow from weekly boss). personally i would say: go for the character you like the most!
note: search for letter farming videos on youtube, the one that worked for me is to afk in the lvl 30 exploration (max 9, if you go for 10 you'll struggle a bit) with both rebecca & nifle deployed as supports (+ as much range as possible

secret letter farm: suzu yun, the video from 9 days ago demonstrates it + he apparently made another video 7 days ago
gold farm: braxophone's video from 5 days ago, run the maze / go fishing