Ever since the option came available when I unlocked the Combat Platforms I've been running around as a custom soldier by the name of Amber Fox.
Recently I managed to win my first S-ranked mission complete, and I immediately got a message that her proficiencies had "increased significantly" as well as a distinguished service medal.
When I looked at her profile again, all her skills had indeed upgraded from mostly C-ranks to straight As.
Now my question is: Is there an upper limit to how far a soldier can be improved in this way? IE, if I keep at it long enough, would she promote to A+ or even S ranks, or will I hit a ceiling and would be better off taking another soldier on missions to rank that one up as well?
(Note: This is somewhat related to the question "How do I make soldiers better?", but the only non-deleted answer it has does not answer my question at all — and in fact it barely answers the question asked, but that's another matter).
Best Answer
A soldier can receive three medals which will increase their stats.
The first is for being on a team that completes a number of dispatch missions.
The second is for successfully completing several missions when deployed as a combat unit player character - Which can be getting an S rank 3 times, or a lower rank 5 times.
The third is given when that soldier deals the killing blow/fultons out an invader from an FOB attack.
Once a character has all three of these medals, their stats will increase no more. It all depends on their base rank as to how far it'll increase their skills, like a C rank can make it to an A rank, A ranks can become A+, and S+ can max out at S++.