Should be doable.
Invisible - either change the combatant game object's layer to a layer the camera isn't displaying or disable the renderers. Both can be done via schematics, Object Visible node for showing/hiding via renderers or Set Object Layer node to change the layer.
As for targeting, that's a bit more complex. Here I'd suggest using a status effect (e.g. a hidden effect to hide it from the player) and check for that effect in Target Conditions of your abilities and items (found in Target Selection Settings > Conditions of them).
Both can be combined by having the status effect use schematics to hide the combatant when applied and shown when the effect is removed via the effect's Change Settings > Schematics.