This page of the trophy guide to Ghost of Tsushima has a detailed description of how to unlock the Cooper Clan Cosplayer trophy. You get it for dressing up as Sly Cooper known from the previous games developed by Sucker Punch studio.
How to get the legendary thief's outfit?
This trophy can be treated as an easter egg. You need to find and equip items that will make Jin look like Sly Cooper. This raccoon is known from his stealth action-adventure games. The Sly Cooper series started in 2002. However, PS4 didn't get any games from this series.
Check the picture above to see how your hero must look to unlock the trophy. The outfit consists of the elements described below.
1) Crooked Kama Headband
Go to the Jogaku Temple.
You must get to the top of the temple's tallest building - the building on the right in the image above. Use the grapple hook to reach the next floors of the building.
The cosmetic item is on the roof of the temple.
2) Wolf of Tsushima
The mask is a reward for finding and praying at all Inari shrines. Jin has to follow foxes to reach the shrines. Locations of all fox dens and Inari shrines can be found in the secrets and collectibles chapter.
3) Gosaku's Armor/Ocean's Guardian
You get the armor by completing The Unbreakable Gosaku mythic tale - check out the walkthrough section.
You can buy the Ocean's Guardian dye from one of the merchants. You get it by spending flowers. Set the blue and yellow dye as active.
4) Sly Tanuki Sword Kit
This sword kit can be unlocked by visiting one of the Pillars of Honor on Tsushima's eastern bank.