I think Hallie is fine with Kellan. As is Cassie. Both pretty names. If you don't like Cassandra but want a longer name, how about Cassia or Cassidy.
Other ideas:
Avery, Alexis, Amber, Astrid, Amy, Bronte, Bonnie, Carys, Cerys, Celest, Delphi, Darcey, Danica, Everly, Elodie, Enya, Freya, Fallon, Faith, Grace, Holly, Heidi, Iris, Ivy, Indie, Josie, Kiki, Kayleigh, Leona, Livia, Lois, Madison, Maia, Maeve, Nia, Nyah, Niamh, Piper, Phoebe, Paige, Quinn, Romy, Robyn, Rose, Saffron, Sasha, Sabrina, Tamara, Tamsin, Veda, Verity, Zelda, Zia, Zara