Yes, Il mentioned about Yona's mother, but never spoke out her name at all, right?
This is pretty peculiar in my view since even Suwon's mother name was given but not Yona's mother once...why? Then I don't know if Il really got married? We only know Yona's mother was murdered.
Yona doesn't seem to have memory about her mother at all. It implies to me Il didn't tell her anything about her mom. It is really really odd, how come he never mentioned a story of his wife to the daughter, well at least tell her how sweet her mother was....something like that.
Could that be her mother was killed by the royal troop and Il took her to the palace due to the priest guidance? Idk, It's possible, up until now Yona probably may not know she is not his daughter if Il took her from her real family since she was baby. (But other sky folks may know?)
If this is true, Suwon killing Il issue can be solved peacefully bwahahaha.
This is a
common trope, I'm afraid. Main characters form high fantasy (and not only) often don't have a parent or both parents, because having such nurturing figures present for them would hinder the plot. The reason why Su-won's mother is given a name is possibly because she might turn out important to the plot (if not the present one, then the past one). She already played a role, if indirectly, as she was the reason Min-su's life was spared. She's also supposed to have lived far away and been sickly, so she can be easily removed from directly influencing the plot and her general absence from the flashbacks is easily explained.
On the other hand, Yona's mother would be a bother for the author. If she were alive, she would either have to run away with Yona (frustrating and difficult to write, and would put her in the leading position instead of Yona and Hak), get killed by Su-won (would paint his character in a truly evil light, since a queen consort can hardly be blamed for the country's state, unlike the king), or get caught and imprisoned by him (again, difficult to write, since the reader would expect to know what's going on with her, even if it weren't interesting, and it would also be expected of Yona to want to come back and rescue her). She would also have to have a presence in the flashback, since unlike Su-won's mother, she would be supposed to live in the Hiryuu Castle.
Together with the many orphaned heroes that had to go on a journey (Frodo Baggins, Luke Skywalker, Harry Potter, friggin' Eragon as much as I hate this Gary Stue...), I take lack of even mentioning Yona's mother's name as a proof that she's not important whatsoever, and we should focus on Yona's present rather than her past. On the contrary, since Su-won's mother's name
is mentioned, I take it as a foreshadowing that his past will be properly examined, which I think we all expect.