To those who never played Sin Alice, Granblue, Fate Grand Order, Elune, Exos Heroes or anything like it:
"R" is for rare in rarity
Normally rarity is like this: Common, Uncommon, Rare, Super/Very Rare/Epic, and SSR or Legendary
Common units are like foot soldiers or militia you can get every stage, uncommon are like archers or roles in the army in some stages, rare are like a squad leader that are in special stages 70-89& chance to get, epic is a commander that can only be gotten through luck as in 10-20%, legendary is like a king or very renowned person you can get 0.5-1%