during the main game, nah. it helps, but it's not necessary and the grinding's a pain. it'll increase the character's aptitudes (meaning more stats from items) and sometimes let them get new skills so it'll be worth it if it's a character you really like using
post game, when you can train like a boss, do it immediately
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"Duel audio? You mean like rap battles?" - termina_rzident
KrahenProphet and Kana are on opposite ends of the Awesome Spectrum.