Young Prince Leo enters the story for the flashback story of how him and young Princess Eva meet.
After Eva gives birth to her first child ‘RoseRed’ the child she conceived with young Prince Henry out of wedlock(see the changeling theory). She is forced by her Royal parents to give up the child to avoid disgrace and humiliation of the Crown. “Its heavier then it looks”- the Crown https://www.youtube.com/watch?v=MVpEq15aqGU
Young Eva must now marry a Prince, the kindhearted Prince Leopold agrees to marry her though he knows of her secret childbirth, and also does fall in love with Princess Eva.
This makes the infamous Family tree look something like this-
Prince Henry + Princess Cora – Wicked Witch, then by the name of Regina
Prince Henry + Princess Eva – ‘RoseRed’ taken away from Eva by Rumple to replace Cora and Henry’s Regina without their knowing
Prince Leopold + Princess Eva – Snow White
-Yes, this does make Regina both halfsister too Snow White as well as stepmother, I understand that people have a problem with that, but I dont agree with it. Regina did not marry a man she was bloodrelated too. Is it twisted, yes! sure it is.