Brigitte is quite a bit different personality wise compared to D.va. D.va has a superiority complex, where as Brigitte seems the most normal among the cast of heroes.
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This should help clear it up:
https://overwatch.gamepedia.com/Mercy
Click on story, end of 1st paragraph
Jeff Kaplan also implied this as well
https://youtu.be/LKg7_qGhWVc?t=17m45s