Chieftain - the Celts were a proud warrior race, so the tribal Chieftain was usually one of the best fighters - but even more important was his strategic ability and command skills. His mere presence was enough to inspire his men, knowing they were being well led by a capable and intelligent warrior.
(I was thinking, since the Celts have a few heavy hitters, of going down the solid defensive unit road - helmeted, mail shirt, and a bogging big shield).
Cost 6
HP 30
Attack 10
Armour 5/4
Speed 3
Resistance 100
Standard Morale Aura, +1, +1/1, range of 4 (only to Celtic units)
Affected by Warpaint and Blacksmithery
Heavy foot, Big shield bearer,
50% bonus Vs Light and Medium Foot Melee, and Archers
100% Vs structures and siege machines.
Heal Bonus x2
(too similar to huscarl...?)
Anyway, let's see what you had in mind and we'll take it from there