I don't think Finland really has a "pop diva", but if you'd ask me, a Finn, the first name that comes to mind is
Anna Abreu. She is half portugese. She sings in English, at least 98% of the time. I don't follow her, but she is very popular so you just know when she releases new music because it's everywhere and a lot of people are talking about it
She's also know to be very sweet and not really a "diva" at all. Her image changed a little some years back to more, in lack of a better word, mature.
From her latest album...
And something older (from 2007), this song made it big...
A few other artists come to mind, but they just don't really fit the "diva" part or what in considered "pop-music" at the moment, but I feel I need to mention them because they are more popular than Anna...
Jenni Vartiainen (sings in Finnish)
Chisu (sings in Finnish)