There are three issues here:
- The easy one. Do you have permission or a license from the artist? Or did they artist release their work under something like Creative Commons? If no, you're stealing their work, don't do it. If yes, move on to...
- The medium difficulty question. Can a person's likeness be used like this? Leaving aside who it is for now, see this question: Is it OK to copy a person's likeness in a realistic portrait without their permission?. tldr; it's thorny legal ground. It's probably okay if the artist got the subject's permission for the work to be used widely.
- The "OMG stop, don't even THINK about doing this" question. Did you say Daniel Radcliffe and the Harry Potter franchise? That is a big, big money franchise with every aspect of it protected and copyrighted that can be.
What you're talking about doing would be classed as feigning affiliation and endorsement from an A-list actor and one of the biggest film and literature franchises there is.
There is a whole cottage industry devoted to trying to rip off big franchises like this in order to boost sales of copycat works, and rights-holders have whole teams of lawyers whose full time job is to find and sue anyone who crosses the line, in order to protect the integrity of their franchise from the hordes of unofficial "me-too"s trying to cash in and cream off 1% of the Harry Potter millions.
I'm sure that's not what you're trying to do, but it's what it'll look like. You don't want to pay that game.
If you just paint a picture and put it on a blog, that's fine, they won't come looking for you. But when you use it in a commercial or semi-commercial context, they have a business interest in making an example of people like you. They will hunt you down, they will find you, and they will sue you.