Board Thread:Speculation House/@comment-24.247.91.107-20140511154823/@comment-25857155-20150212095140

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My theory? Raven left either because Taiyang cheated on her with Summer, or because they had a three-way thing going and she decided she didn't like it anymore when Yang was born. I lean toward the first one. In either case, I suspect Yang has her father's name because her mother left immediately after she was born. Ruby has her mother's name because she and Taiyang were not 'together' when she was born. Qrow, Summer, Raven, and Taiyang were on the same team together at Beacon. Qrow Branwen is Raven Branwen's brother, thus, "that's my Uncle Qrow!" from Ruby, and making him Taiyang's brother-in-law, assuming he and Raven actually married.

Another possibility is that Raven learned about something she "shouldn't have," and left to protect the ones she loved (Taiyang, Yang, and Summer, since Qrow could handle himself). In this theory, she fails, leading to Summer's eventual death several years later. I rather doubt that their situation was a "normal" thing, if Taiyang's apparent heartbreak is anything to judge by. I doubt a Huntsman would be unprepared for such a thing if it were the norm.