Hi I don't exactly remember how you get the skin but try to 1 act the last act that has 2 mission with Batista marcus and if that doesn't I think it might have been an limited time event.
Not exactly sure to be honest I did paid for all the chrome and black steels. 15$ for hivebuster mac, paid 20$ for ape kait,50$, paid 20$ for Greece and rev 9, and all other guest characters were free from my gamepass.