xcom-2xcom-2-war-of-the-chosen
I'm on a guerilla op right now, and I'm seeing a seventh soldier in my squad that I don't remember adding to my squad.
They have no class (no, not even Rookie), seem to have a randomized name, and the following stats:
Who is this, why are they in my squad, and will I keep them once I'm done with the mission?
To clarify, I remember setting up the squad clearly, but it's a somewhat old save so I could have done something previously that I don't remember.
This might be anecdotal, but yes, you can:
I ranked all my squad members to Colonel on the downed Avenger mission, by simply refusing to destroy the signal beacon and farming the incoming assault drops by setting up Overwatch traps.
Took me an hour to do so. After returning to the Avenger I was greeted with multiple rank ups after promoting the soldier (meaning went all the way from Squaddie to Colonel)
Alright, I dug into the code some more and updated my answer.
Here are the number of kills needed for each level, taken from DefaultGameData_XPData.ini:
Rookie Veteran Commander Legend
Squaddie | 1 | 1 | 1 | 1
Corporal | 5 | 6 | 6 | 8
Sergeant | 12 | 12 | 12 | 18
Lieutenant | 24 | 24 | 24 | 40
Captain | 38 | 38 | 42 | 70
Major | 57 | 57 | 60 | 110
Colonel | 81 | 81 | 85 | 175
However, there's more to it than just straight-up kills.
Every time a unit gets a kill, all the other units on that mission get partial-kills. The game calls these "KillAssists", but that's a misleading name because the unit earns a KillAssist whether or not they did any damage.
How much a "KillAssist" is worth varies by class
(taken from DefaultClassData.ini, variable name KillAssistsPerKill)
Specialist: 1/3rd kill Psi Operative: 1/3rd kill Rookie: 1/4th kill Ranger: 1/4th kill Sharpshooter: 1/5th kill Grenadier: 1/5th kill
Additionally, Psi Operatives gets a kill bonus called a "PsiCredit", worth 1/3rd kill, every time they are hit by Feedback.
Finally, if a unit started at any rank above 'Rookie', they'll have additional hidden bonus kills equal to the number necessary for that rank.
Assuming you obtained a unit at 'Rookie', you can see how many kills they have in the armory...
but without a save-file viewer (which doesn't exist AFAIK) you cannot view their KillAssists. The best we can do is guesstimate from the number of missions.
In my screenshot above, if we assume she's encountered, say, 8 enemies per mission on average, then over 10 missions she's encountered 10*8=80 enemies. That gives her 80-29=51 KillAssists, which counts for 51/4=12 kills, bringing her approximate kill-total to 29+12=41 kills. So she needs around 16 kills or 64 KillAssists to level up to Major.
Edit: There is now a mod that shows KillAssists on the soldier info screen: Accurate Soldier XP
Some other interesting things I discovered while code-diving:
KillAssists and PsiCredits are not boosted by Wet Works.bUseFullXpSystem=false). Though, I suppose you could consider KillAssists and PsiCredits a form of XP...KillAssists). There's an option to allow them to level up outside of missions as well.
Best Answer
If you have the bonus called Volunteer Army (either from continent bonus or resistance order), there will be a chance of a unclassed resistance soldier join you on missions.
There's another similar bonus called Double Agent where there's a chance of a random advent unit (stun lancer or trooper) join you on a mission.