With a few mods and some tiny config file changes, you can make sure this class is only ever used by Mercy from your character pool. Just be warned that you'll probably have to do these changes every time the mod updates.

First off, you'll need the following mods:

• RuNis and E3245’s [WotC] Overwatch Mercy Squadmate.
  
• Casadoom’s Overwatch – Mercy Voice Pack.
  
• krj12's WOTC Use My Class. This is the most important one as it forces the game to assign the right class to your character pool soldiers when you acquire them.
  
• While not necessary for this tutorial, I also recommend RealityMachina's Uniformity mod if you have a bunch of cosmetic mods.

Open up your C drive and go to the file path
You'll know you're in the right place if the folder has a file called MercyClassMod.XComMod. Once you're in there, go to the Config folder, then open up the XComClassData file in a text editor of your choice. You should see something like this at the top of the file:


NumInForcedDeck and NumInDeck are what we want to change. Set both of those numbers to 0, then save the file. (For those wondering, setting these numbers to 0 is what removes the class from the random pool the game uses when promoting rookies and such. However, thanks to the Use My Class mod, the game can still assign it to soldiers who are specified to use it.)

Next, start the game and create Mercy in your character pool.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1398846545

Make sure the soldier type, soldier class, and appearance settings are identical to that of the image above. Also make sure your character pool usage isn't set to Random Only, otherwise none of your custom soldiers will be used in the game.

And that's it! With luck, you'll soon find the valkyrie Mercy in your game, whether it be through your starting rookies, soldier recruitment, or mission/black market rewards.

For the pack I recommended, the "Heroes never die!" voice line is attached to the cue for the specialist's Restoration ability. By default Resurrect doesn't use this cue as it sounds odd with any pack that's not Mercy's, but if you want it to use it, go to the Src folder of my mod and open X2Ability_MercyClassMod. Find the function and the line inside it, then change HealingAlly to RestorativeMist (keep the quotes in there though).