optional characters are only available in the second half of the game, after the floating continent. BEFORE YOU LEAVE THE FLOATING CONTINENT there is an option to wait. You have to do this if you want shadow in the second half of the game.

As for stats, stamina is useless and if you plan on going to lvl99 then you'll reach approximately 9999 without hp boosting anyway

Also Sabin's Blitzes use the magic stat to calculate damage, not vigor/strength