Do you mean taking other peoples’ work and putting it inside your demo reel? Or copying a pre-existing character and featuring it in your reel? The first one is a definite no-no. Prospective employers/clients will be watching your reel expecting all the work to be yours and yours alone. The second… well, to be honest, both ideas seem wrong to me. You want your reel to be about you, not about somebody else, and not a concept that you copied and put in your reel.
There’s nothing wrong with picking apart professional work and looking to see how they did it. There’s nothing wrong with building a model that looks identical to it. But putting it inside your demo reel? I’d shy away from that, but that’s just me. Ideally, your reel is about you. It’s your introduction to the person you want to hire you. If you put a model of Lara Croft in there, even if you built it yourself, it’d seem like you’re uninspired at best, a plagiarist at worst, and either one will probably kill your chances of being hired in the face of a reel that features original character design and models.
Again, that’s just my suggestion. There are many artists who are very successful who started out making copies of stuff. And everyone needs to get inspiration from something, but the very successful artists almost all have exciting, visually stunning reels plumb full of awesome, original stuff of their own creation.
Other than that, keep it up, you’re better at character modelling than a lot of us, and there’s a market for low poly stuff! =)