I can't lose them too... Not again. The following article contains significant spoilers and/or ending details of Valkyria Chronicles 4. If you don't want to get shot, tread carefully!
This is the Valkyria Chronicles 4 ace list. Don't say we didn't warn you.
All Aces in Valkyria Chronicles 4 are Imperial soldiers, though Secret Agent Saki appears to be an undercover agent actually employed by some country in the Far East. Unlike VC1 and VC3, many enemy aces also provide a CP for their faction and thus count as both a leader and an ace in the after mission report.
Something of note is that several Aces encountered throughout carry modified weapons with adjusted stats compared to their standard Imperial-only counterparts. The stats of the weapon they unlock do not necessarily match the one they use: indeed, some such as Cova the Peacock do not even unlock the same type of weapon they use (in his case he uses a VB GW 2 general-purpose Mortar but unlocks a VB GW 2B(e) anti-tank Mortar).
There is also a pseudo-Ace, Major Burke, who commands a Heavy Imperial Tank which uses his name as the unit name, but he is not counted as an Ace kill and does not have an item drop.
DLC weapons are not required to earn the "Excellence in Armament" Medal.
The DLCs "A United Front With Squad 7," "The Two Valkyria" and "Edy's Advanced Ops" do not contain Aces.
Aces with a * next to their name indicate they are leaders and thus add an additional CP to their forces.
Scout
Shocktrooper
Grenadier Paragon
Sniper
Tank Commander
Lancer