It's taken for granted that Roxy is now an official part of the Winx. Why? Maybe because she flew back to Magix with them. However, lets keep in mind that Roxy was going to Magix to study in Alfea while (recall Season 4 Episode 1) the Winx would be teachers in Alfea. Roxy is also 3 years younger than the other Winx and has no magical experience at all!
Some people say that Roxy is part of the Winx because she has believix. But does she really have believix? The first thing that I notice about Roxy's 'believix' is that her transformation had music similar to the believix music but not the same music as the other Winx girls...of course, that may be because she wasn't part of the Winx (remember the first time Layla transformed the music was only an instrumental). But I think it is because her transformation wasn't really believix! I think it was magic Winx! Want more reasons? Take a look at the picture of Roxy's transformation:
Look! Just like in the Winx transformation (the first transformation), her clothes are one colour (green). Notice that her wings aren't as big as the other Winx's believix wings. Her shoes aren't the same Converse-like shoes that the other Winx have in believix. Her's have no laces and look like boots. In which transformation was the shoes boots? Thats right, Winx! Unlike believix, absolutely nothing has happened to her hair, not even a single braid or tiara. In Winx transformation, most of the Winx (exception Stella [tied up hair]) had nothing happen to their hair! What does that show? Plus, notice the background of the transformation is just like the Winx transformation background!
What else? Well, nothing more in the picture, but why can't Roxy use Speedix, Zoomix and Tracix or transform into Lovix and Sophiex? Some people would say that she willed herself into transforming (and technically this is true because for the Winx transformation you have to will yourself into transforming [remember Bloom transforming for the first time in Season 1 Episode 3] and you still have full power) so, like when Bloom willed herself into Enchantix, she doesn't have full power. Well, when Bloom saved her kingdom and people, she got full Enchantix powers. Likewise, when Roxy got a human to believe in her, she should get full believix! Well, after Roxy transformed, her father believed in her! So basically, she should have full Believix powers...if she has believix. I think she has Winx!
Why would Roxy be in the Winx then? Well, maybe since Bloom (the unofficial leader of the Winx Club [remember Season 2 Episode 23 Tecna said 'give us the word Bloom, which indicated leadership]) related to her, and felt she needed help in this whole magic world. Bloom understands what it's like being the 'new fairy'
What is your opinion?