Hameln

Hameln is a support who generates arithmetica and cards. When roused, he gives you extra 2 arithmetica per turn. His Memory Rondo also makes a lot of Insights (0-cost cards that give 1 arithmetica and draw a card).

He works well as a keyflare bot, since you can spend arithmetica on Memory Rondo and immediately refund it with the Insights generated.

Hameln’s exalt can apply Rondino to command cards to double their effects. This lets you get huge value out of Memory Rondo or other cards like GLotan’s Long Blade: Perish. At E2, it also gives an arithmetica discount.

Soul Overture inflicts random debuffs on enemies, which can inflict weakness and vulnerable if you're lucky (or retry over and over).

Hameln is fairly powerful in Faded Legacy stages, as if you put him with other card draw and arithmetica supports, he can loop Memory Rondo and set up near-infinite combo turns. He falls off in Astral Reign because Arithmetica Harmony limits the number of cards you can play each turn.