Any character in FFXIV that's been included in an optional side quest is effectively immortal. Other games have done it diffeᩚᩚᩚᩚᩚᩚ⁤⁤⁤⁤ᩚ⁤⁤⁤⁤ᩚ⁤⁤⁤⁤ᩚ𒀱ᩚᩚᩚrently, but in FFXIV it seems that we need to be able to "canonically" do any sidequest at any time, so killing off someone who's been involved in them isn't possible.

So with that in mind, we can guarantee the safety of Estinien (dragoon questline), Krile (Eureka), Alphinaud and Alisaie (Coil of Bahamut), and if they're optional, any of the quest givers for the ARR hard mode primals (so Urianger, Thancred, Yshtola). But those are just the ones I remember, if a character like G'raha popped up in a random side quest in the darkest c﷽orner of the map for half a second one time, even that would disqualify them from being killed off.