Can I take a fortnight character someone else spent money on.
That sounds cruel. Why would you take someone else's Fortnite player?
I need it for a roleplay I'm doing.
You shouldn't steal other people's characters. It's likely that this violates the Terms of Service or Fair Use which can result in your account being banned. There are a lot of free assets that you can use for roleplaying instead.
But I need my roleplay to be great!
You don't need to steal something that belongs to someone else to make your roleplay great. I'm sure there are many free assets that you can use for your roleplay. Just be creative!