Some information may be missing, as sometimes I have to go back and watch the movies just to get everyone. Bear with me, it will all come together eventually.
Romantic Interests
Prince Eric (Barbie in the Nutcracker)
Prince Stefan (Barbie as Rapunzel)
Prince Daniel (Barbie of Swan Lake)
Prince Dominick, Julian (Barbie as the Princess and the Pauper)
Aiden (Barbie and the Magic of Pegasus)
Derek (Barbie in the 12 Dancing Princesses)
Prince Antonio (Barbie as the Island Princess)
Jeremy, Ian (Barbie and the Diamond Castle)
Relatives
Tommy (brother, Barbie in the Nutcracker)
Grandfather Drosselmeyer (Barbie in the Nutcracker)
Aunt Drosselmeyer (Barbie in the Nutcracker)
King Wilhelm (father, Barbie as Rapunzel)
Queen Genevieve (mother, Barbie as the Princess and the Pauper)
Brietta (sister, Barbie and the Magic of Pegasus)
Ashlyn, Blair, Courtney, Delia, Edeline, Fallon, Hadley, Isla, Janessa, Kathleen, Lacey (sisters, Barbie in the 12 Dancing Princesses)
King Randolph (father, Barbie in the 12 Dancing Princesses)
Animal Friends & Villian Familiars
Pimm (familiar, Nutcracker)
Penelope, Hobie (Rapunzel)
Otto (familiar, Rapunzel)
Lila (Swan Lake)
Serafina (Princess and the Pauper)
Shivers (Pegasus)
Brutus (familiar, Dancing Princesses)
Twyla (Dancing Princesses)
Slyder (familiar, Barbie and the Diamond Castle)
Sparkles, Lily (Barbie and the Diamond Castle)
Villians
Mouse King (Nutcracker)
Gothel (Rapunzel)
Rothbart (Swan Lake)
Wenlock (Pegasus)
Preminger (Princess and the Pauper)
Duchess Rowena (Dancing Princesses)
Queen Ariana (Island Princess)
Lydia (Diamond Castle)