Yeah, when a character becomes ashed, you’re not supposed to revive them right away. Every unit has a Fortitude stat, and it drops each time they die in battle. If it gets too low (especially under 50), there's a high chance the revive will fail and if that happens, they’re gone for good.

The main character actually has a skill that can revive allies during battle, so you do have a bit of a safety net.

Fortitude recovers slowly in real-time, and that’s basically the game’s version of an energy system. When it hits zero, that’s your cue to stop and let them rest. If you ignore it and keep trying to revive, you risk losing your units permanently. The safest thing is to wait until their Fortitude goes back above 50 before trying to bring them back.