It's not too complicated to add a promotion for Chad in Nightmare. For the most part, you can just copy his thief data onto another class, preferably unused. You would have to add functionality to promote Thieves to some promotion item, but that's not difficult either.
In my hack, I made Thieves and Soldiers promote into themselves :D
You get a Lv20 theif, then promote it. It doesn't get any promotion stat-ups, but now it's a Lv1 theif with the stats of a Lv20 that gets and almost instant level-up for every Lv17 Fighter it kills. 1 chapter later, he's back at Lv13 with 12 level-ups worth of extra stats :D
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