It's an addon mod that connects Mns mercenary with MCA Reborn.

You can bring villager skins from MCA Skin Library and apply them to your mercenaries. Also the skin can be imported by nicknames.

It is basically client-side. It works fine as client-only.

If the mod is also installed on the server, it will sync data so players with the mod can see each other's custom merc skins. Even if a player doesn't have the mod, they simply won't see the skins and no issues or crashes will occur. (Since only player merc class NBT and skin IDs are stored on the server, network and storage impact will be minimal.) Skins are applied on each merc class separately, and saved per server(or world).

Default keybind for GUI is "Numpad -"

You can also access it by clicking the golden armor to the left of the tooltip on the Merc screen.

To apply skins, click sword icon on MCA Skin Library.

Or enter a nickname in the GUI textbox and click the check button to generate and apply it as a Skin layer.

Each skin is categorized into three layers (skin, clothing, and hair) by automatically recognizing the number of pixels. Each layer can be applied individually and stacked on top of each other. Of course, since this automatic recognition isn't always perfect, you can click layer name button to change it yourself. (Because each layer has a scale difference of about 1 pixel, if clothing clips with the skin, you can prevent overlap by moving the clothing up to the hair layer.) Also Armors can be toggled with top right armor icon. (I know you can just transmog armors. However I don't want to transmog each thing everytime. Yeah, it's one button toggle. Who wouldn't like it?) You can clear every cache and history in debug menu. (Top right "D" button on GUI) Say goodbye to ugly Squiward mercenaries