Speaking of the Talisman, let's talk about who might need it. I'll be listing in my personal order of preference.
Miguel: Currently has 10 RES, and a 40% growth rate, but his low, low speed means he gets doubled, and his growths suggest that will continue to be the case. The talisman saves him more HP per turn than anyone else. On the other hand, we generally don't throw Miguel at mages.
Cyra: 3 RES and 25% growth is not a great combination, especially for a front line class like mercenaries. She has good speed and luck though, so she may be doubling enemies rather than being doubled.
Grifen: The peculiarities of crossbows means that Grifen has a tendency to be aimed directly at enemies who are magically inclined. And with 8 RES and only a 25% growth, she has a lot of room for improvement for her magical defenses. On the other hand, she's about to promote, and may find many of her turns spent healing people soon. Well, moreso than now, anyway.
Below this point, we're reaching situations where I personally would advocate against giving the talisman to the character in question.
Ciera: Wind magic is effective against her, so having a bit more RES to soak that up would be useful. On the other hand, she has 13 RES already, and just shy of 50% growth for it (better than Miguel or Grifen)
Julius: At 4 RES and a measly 10% growth, Julius can be expected to be more susceptible to magic than anyone else on the team come the end-game. That said, he's usually too busy to actually be at the front, so the difference it would make for him would likely be negligible.
Sheila: at 9 RES and 35% growth, the idea that Sheila could cap RES is actually impossible. She sticks to the front, and fights mages regularly. However, Sheila has fantastic speed, tons of hit points, and strength and skill to reliably chop mages in half. It would help her, but there are others who it would make a greater difference for.
Below this point, I will actively campaign against it.
Fezz: Despite low growth at 30%, Fezz has 15 RES. More to the point, he generally fights physical units. Briefly.
Candy/Alec: If we're relying on the ability of our performers to tank, we have done something very, very wrong. Next.
Efa: Has stellar RES already, and routinely attacks from outside a mage's capacity to counter in the first place. No.
Leo: AHAHAHAHAHA no.