If you are expecting Korean Ninja, sorry not.
This is rather a proof of there was no Ninja in Korea.
Annals of Joseon Dynasty
Jungjong annals 15
Jungjong 7th May 5th (1512 May)
This record was written 2 years after Sanbo-No-Ran (Riot by Japanese settlers in Korea)
The Japanese prisoner Yoshira (要時羅) testified to the interpreter
"I tell you everything I know because I'm gonna have to be Korean rest of my life.
The Japanese whose family members were killed during the riot asked the lord of Tsushima (So clan) to send troops.
The lord said if they don't make peace, he would plan to send troops to Koje(巨濟).
It is obvious they will come within this year.
It is only 48soku from Tsushima to here, Koreans won't be able to defend if they land during dawn.
And what is more, there are Shinomi(時老未) in Hakata（花加大）.
They are good at disguise. They can sneak into enemy camp like mouse, escape like bird.
No guard can notice.
Tsushima recruited them to set fire on camps.
I heard people from Tsushima talking about it."
The testimony was reported to King and King told to The Military Affairs.
The Military Affairs told to King "What Yoshira said about Tsushima brings people good at disguise from Hakata is ridiculous, it's obvious the story is for scaring us.
But asking Tsushima to take revenge seems to be true. It is better to order to secure defence."
According to Professor Shosuke Murai (Tokyo University), Yoshira is Yoshiro(与四郎) , Shinomi is Shinobi. This testimony proves there were Shinobi in Hakata area in those days. (Of course Hakata is Kyushu Japan) And that means Sengoku war lords were hiring Shinobi.
漆原縣囚倭人 要時羅, 語通事曰: "我等永爲朝鮮人必矣, 本道凡事, 何不直言哉? 前日接戰時, 死倭子壻弟姪等, 請於島主, 請兵隣島, 攻朝鮮, 島主曰: ‘若不永和, 則先攻巨濟事議之。’ 今年內出兵來攻明矣。 自對馬島, 距此境四十八息。 夜暗無月, 乘昏發船, 侵晨下陸, 則朝鮮人未及禦敵矣。 不獨此也, 深遠之國花加大國所在時老未者, 能變作形體, 陣中入歸時則似鼠, 還出時則如烏鳥, 變行無窮, 雖稠人列在左右, 不得解見。 欲求請此人, 城柵屋舍焚燒設計事, 對馬之人, 紛紛開說。 予乃聞知而出來。" 云 啓下兵曹。 兵曹啓, "要時羅等言, 島主欲請花加大國能變形體人助戰之說, 恐動明白, 且涉怪誕矣。然接戰時敗死人子壻弟姪等, 報復設計, 巨濟鎭爲先攻擊事, 島主前請說等語, 似爲不虛, 防備諸事, 益加謹愼事, 左右道移文何如?" 允之。
The record also shows Koreans had no idea what Shinobi was.