Fire and Ice
I believe Elsa and Merida represent two elements as in ice and fire. Elsa is the Snow Queen so she is naturally fit for the role of ice and Merida with her fiery red hair and equally inflammable personality is perfect for fire. However, it is unique that both have an independent streak when it comes to men. They refuse to even speak of the topic or acknowledge their feelings about anybody, that is to say if they even have somebody in mind. Also they are both courageous in two different ways. Elsa is fearless because she faces her fears regarding acceptance of herself and self love while Merida is naturally a stereotypically fearless red head, riding fiercely around on horseback shooting arrows right in the bullseye. Moreover, the theme of family and love is also apparent in both movies. Plus the fact that both the characters are flawed in some way which ends in tragedy but they figure things out in the end. I found the contrast between the two movies quite interesting as well. Frozen is based on a rather blue setting while Brave is more on the warmer end of the spectrum. Also Elsa is driven more by fear of herself and her powers and Merida by rage of being forced to marry. That's all I can come up with for now. Thank you for the challenge!