That is tough, man! You sure we don't need math skills for this?
Anyway, what the hey, I'll take a stab:
5, 10, 15... that's the toughest one IMHO, I guess it's the Lvl, Exp, CT, or height you can hit with a math skill multiple of 5? Or it could be the progressive accumulation of Brave pts you get from using Cheer Up over and over.
13, the number of Zodiac Stones.
31, Rafa's orginal Brave score, Malak's original Faith, or (Most likely what you were looking for) the number of Treasures you can win from the Propositions.
48, the number of different Monster classes (Not including Demons and Apandas).
108, the number of beads in the accessory '108 gems'.
999, max damage or max h.p. (and I imagine you could also max a character's m.p. to 999).
Close enough??
