RELATIONSHIP INFO
Birth order: Goryeo era: First. However, since he had half-brothers from his father's side, counting his half-brothers, third.
Who does s/he take after: Father, physically.
Relationship Status: Goryeo era: married. However, modern era, single.
Taejo Wang Geon
Wang Kǒn - father - FC: Jo Min-Ki
Status: Dead.
Goryeo Era.
Jeon Iseul
from Hangul 이슬 which means "dew" - father - FC: Park Ji-Il
Status: Alive.
Modern Era.
Wang Mu
Hyejong of Goryeo - former crown prince - half-brother - FC: Kim San-Ho
Queen Sinmyeongsunseong of the Chungju Yu clan
Queen Yoo- mother - FC: Park Ji-Young
Status: Died 2 years after Yo died.
Goryeo Era.
Jeon Yun-Seo
from Sino-Korean 尹 (yun) meaning "govern" combined with 序 (seo) meaning "series, sequence" - mother - FC: Lee Mi-Sook
Status: Alive.
Modern Era.
Wang Won
9th Prince - half-brother - FC: Yoon Sun-Woo
Wang Eun
10th prince - half-brother - FC: Byun Baek-Hyun
Wang Jung
14th prince - brother - FC: Ji Soo
Wang Wook
8th prince - half-brother - FC: Kang Ha-Neul
Baek Ah
13th prince - half-brother - FC: Nam Joo-Hyuk
Wang So
Gwangjong of Goryeo/successor of Wang Yo/4th prince - brother - FC: Lee Joon-Gi
Princess Hwangbo Yeon Hwa
Queen Daemok - half-sister - Wang So's future consort - FC: Kang Han-Na
Sibling(s) Status: Goryeo era: I won't spoil names, but let's say Yo killed a few of his brothers.
Modern era: None.
(c) - GIFs: here.
Lady Gyeong Hwa
Wang Mu's daughter - half-niece - brother's consort - FC: Jang Seo-Hee
Unnamed Lady
Wang So's and Hae Soo's daughter - half-niece - FC: Seo Eun-Sol
Choi Ji Mong
king's astronomer/Wang Mu's best friend/used to be Yo's father figure, now enemy - FC: Kim Sung-Kyun
Lady Bok Soon
Wang Wook's daughter - half-niece - FC: Kim Do-Hye
Park Young Gyu
In Goryeo Era: father-in-law - FC: Choi Byung-Mo
Queen Mungong
of the Suncheon Park clan - consort (FC: Sung Yu-Ri)
Best friend(s): Wang Wook, however, growing up, they drifted apart. In modern era, he has none.
Good Friends: -
Acquaintances: -
Pets/Special Animals: -
Note: In the modern era, he has no siblings, no nieces, etc.
Main Enemy(ies): In the Goryeo era: Wang So, Hae-Soo/Go Ha Jin, Wang Mu.
Most Hated: In the Goryeo era: Wang So.
Queen Munseong
of the Suncheon Park clan - consort (FC: Im Yoon-Ah)
Lady Cheongju Nam Won
of the Cheongju Kim clan - consort (FC: Kim So-Hyun)
Children: From his second wife: Prince Gyeongchunwon and an unknown princess. Yo wasn't alive when his wife had given birth to their baby girl.
Children's Statuses: Both dead. The princess died from a fatal illness while Gyeongchunwon was killed by Yo's brother (Gwangjong/4th prince Wang So) after being suspected of treason.
[Author's note: This actually wasn't something I made up. This, historically, happened, proving that Wang So had really lost his mind while Hae-Soo was no longer there - he had no proof to accuse Yo's son of treason and yet he killed him, regardless ]
[ Author's note 2: No spouses or children in the modern era ]