Golden Sovereign
From the MEGA Wiki
The Golden Sovereign is an outfit cosmetic for your Titan. It's unlocked by the Unstoppable achievement: Win 20 matches.
Equipping
In the lobby, pick a Titan and click Customize, then find the Golden Sovereign under Outfits and click Equip Item. Each Titan keeps its own look, so it has to be equipped per Titan. Until it's unlocked it can be previewed but not equipped.
Other outfits
Cosmetics
Unlocking
City Cap
Frontier Titan
Builder's Hard Hat
Iron Chef
Party Animal
Neighbourhood Beanie
High Society
Monarch
Titan Shades
Champion Shades