https://www.gamespot.com/articles/street-fighter-5-starts-kens-monster-hunter-world-/1100-6457483/

Street Fighter V has begun its Monster Hunter World crossover event, offering a rad alternate Ken costume in exchange for going on a very Monster-Hunter-like quest for armor pieces. The ultimate result will be a Rathalos Armor set, but you'll have to take part in special events every week through April 12 to unlock it all.

Completing the armor will work much like it does in Monster Hunter World. You'll need to collect each piece to finish the whole set. Each piece will be available in Extra Battle Mode on a rotating basis every week. This week, the boots are available. Next it will be the gauntlets, then chest-piece, and finally, the helmet. Once you've gotten all four pieces you'll unlock the alternate costume.