Just finished unlocking all of the characters, around 70,000 rhythmia. If you want to farm shards you don't have, Libra is your friend. I was down to only needing black shards, and that shit saved me sooo much time.
Also, Karb, the "Scarlet guy" is a chick. I honestly thought it was Edgar at first, and then I went "wtf, she didn't have blonde hair in the game".
For anyone that's curious about farming shards too, certain ones only show up in certain level ranges. Not blanking out character names, so I'll put it in a spoiler tag just in case anyone cares.
Type: Levels
A: 1-25
B: 1-75
C: 26-99
D: 51-99
Character - Crystal color - Crystal type
Princess Sarah - Gold - C
Minwu - Silver - B
Cid - Yellow - A
Rydia - Green - B
Kain - Navy - D
Farris - Crimson - B
Locke - Indigo - A
Aerith - Red - B
Sephiroth - Black - D
Seifer - Grey - C
Vivi - Orange - A
Yuna - Turquoise - B
Prishe - Purple - C
Ashe - Scarlet - A
Snow - White - C
Cosmos - Rainbow - C
And yes, that does mean that if you over-level your characters too much, you can be locked out of getting Locke/Cid/Ashe/Vivi.
Easiest way to farm dark notes is deleting your newest, incomplete dark note, then running an old one with a party stacked with Libra, that averages out to the level range you want the dark note to be. With a full Libra group, you'll see 7/9 items when the dark note unlocks, and all three bosses (if you're looking for Ozma/Chaos/Shinryu/Omega for the trophy). Whenever you finish a dark note, the game checks if you have any incomplete notes, and if not it generates a new one based on the average level of the party that completed the previous one.
Helps to save a toooon of time farming shards.