Why Is It Called Rebus in Japan and Kartia in the West?
In the fiction, Rebus is the world itself, a land whose civilization runs on Kartia, cards inscribed with texts in a divine script. Write the right words on a card and the thing written becomes real. The Japanese title names the stage; the Western titles name the tool, and Europe added a Legend to make it sound like the fantasy epic it also is.
The word Rebus carries the game's thesis. Atlus explained in a 1997 Famitsu interview that in the ancient Roman tongue it means combination, or joining together: the game is about combining single written characters into stronger texts, weapons, spells, and creatures. The same interview was careful to say the idea was not borrowed from Megami Tensei's demon fusion, the other famous combination system in the building.