We got two red sexlinks and three black ones (none of them lay yet) but we don't know what kind of chicken this is
Agree it's the male counterpart of your red sex link hens. He's classic red sex link rooster. If you don't plan to keep a rooster, I'd start making plans about his future before he gets too noisy.
The stance looks like a rooster giving you the stink eye, about to ruffle his neck feathers. It's comb also stands straight up, so I'd think it is a roo. Roos are more colorful and have longer tail feathers than hens. PS - I don't know much about sex links, but I just did some reading, and the male red sex-link is supposed to be mostly white (like yours). So I'm not sure how it slipped into a batch of pullets.