There's a fish in Densetsu no Stafi 4 called Lovely, who sells (from what I remember) CDs of certain tracks within the game itself. If you buy everything, she sends you a little photograph:
Not much, but I thought it was cute.
There's also the secret final boss which you can only fight by replaying the levels and doing the hidden post-game puzzles. She blows heart-shaped kisses that swallow Starfy/Starly up and turn to stone, preventing them from doing much other than moving left and right. You can check it out on my twitter here:
https://twitter.com/scooblyboob/status/1515234093151178754
https://twitter.com/scooblyboob/status/1515234124495302657