Though she is usually proud and self-centered, Stella also cares deeply about her friends and family, not just herself. She will stick to the people she loves and do anything to keep them safe. Stella is not always seen wearing a smile on her face, but she does crack jokes to cheer her friends when they are depressed. Basically, Stella has a cheerful and optimistic personality.

Besides that, she is totally obsessed with fashion; which clothes look best on her, and which dresses match her face the best. Stella often gives fashion advice to people in Alfea, but this advice is often not taken well. She would rather flirt with Brandon than study and do work, which sometimes annoys the other girls. The most intuitive of all the members, Stella likes to match make but has often had problems with understanding why her friends do not always like getting set up.

However, despite her intuitions, Stella can be rather oblivious at times. She was sent to Alfea to become a stronger fairy, but this did not mean so much to her, though she did effectively become a fully accomplished fairy and a powerful one by fighting the first three season villains with the Winx. She uses her magic for anything, but she mostly wants to help her friends as deep inside she is a generous and helpful girl. Although she might disagree with her friends, Stella is very supportive and protective and will not hesitate to help them. She learns to accept that love and friendship are far more important than royalty, fashion, and shopping. Sometimes she hides her true personality because of her upbringing. In short, her cheerful exterior masks great sorrow.

Source: Winx Club Wiki

Note: You do not need to sound like the Season 4 Voice Actress. Do whatever you think fits Stella best! :)