If you mean the one that you use in the "Trainer" position--nothing matters there. Currently all Forsaken are equally good at doing the Trainer job. I just put one there that I don't often or ever use for combat, since the position doesn't regenerate stamina.
I like to make use of exactly 1 Rampant Forsaken. My preference is for her to be Undead so that I recoup 90% energy cost when she is deployed from the "Energy Raider" trait. There's a balance of abilities that I use to make her generally useful at each place in a loop--bonuses to Trauma so that she can deal enough damage to get Forsaken into 2EE downtimes, self-buffing Punisher breaks so that when I have lots of EE at the end of a loop she gets significant buffs, which leads to solid damage in the middle of a loop to 2 different stats.
I've never tried to beat Victory (loop 10 or 15) without Forsaken but I'm pretty sure it's possible, though it could be difficult. You'll want some well-trained Forsaken for that fight especially if you want to get a Distortion on her. You'll also want Forsaken when you start going against Devil Chosen (loop 11 or 16 probably). You can't trigger most of their T3 moves without a Forsaken.
Personally I just like using them. Loop 1 I'll get 1 or 2 Forsaken that give a bonus to a Circumstance damage as their Punisher. Loop 3 (I skip Loop 2) I'll use those Punishers to set up the Superior Chosen. So I'll start using Forsaken in the damage-dealing portions of the loop on Loop 4 or 5 (depending on whether I skip Loop 4 or not; I usually do).
