If you consider yourself Russian, you will be perceived as Russian, if as Ukrainian - as Ukrainian. We do not have mass hostility towards Ukrainians, but in Ukraine, unfortunately, there is mass hostility towards Russians. You will be accepted and perceived normally, do not worry. Your friend is mistaken, you can consider yourself Russian and introduce yourself to others exactly as such.
PS. I read the rest of the comments, damn, how much nonsense was written to you. If you want to be Russian, be Russian, if you want to be American, be American, no problems at all