Currently, there are some characters that that should be Legendary but they are not, some are especially very important to certain characters, and in 3K they might end up dying in the first few turns due to them being Generic.

This might very well be a bug that CA have missed, and these characters are very likely to receive their Unique portrait/ being in a DLC later. All of these characters have their own Unique traits given by CA

They also have their own unique model/story on Dynasty Warriors
On the table here found on the PFM, Generals have a value of auto_survive_threshold here of 50

For Generic characters, if they are not captured, the chance of them getting killed is rather high, and since it's generally the AI who kills them, chances are they won't appear on your campaign again, and you are at the mercy of the AI, should the AI choose to kill them, they are gone forever

By making them Legendary, it will make it easier for you to run into these characters later/ reduce the chance of them getting killed, as the AI might be more willing to hire them instead of executing them

Sun Quan, unique portrait + being the Ruler of Wu, he starts off without the Legendary Status, and Sun Ren, on the other hand, starts off as a Legendary

Sun Quan

Xu Huang
Being one of the Five Elite Generals for Wei, he starts off without the Legendary status and stationed in Shanyang, serving the Han Empire, within the first 10-15 turns it's very likely he will end up dying due to the aggressive nature of the Han Empire using him. He is one of the main characters for Cao Cao late game

Xu Huang

Xun Yu
Xun Yu starts off being the first turn fodder for Zheng Jiang, if Zheng Jiang decides not to recruit he will be dead on Turn 1, you are completely at the mercy of the AI. With him being Legendary hopefully, Zheng Jiang will end up recruiting him which makes appear on late game easier
"Xun Yu's contributions to Cao Cao's forces and administration are immense. On one hand he recommended many other men of calibre to Cao Cao, including Xun You (his second cousin-nephew), Chen Qun, Zhong Yao, Guo Jia and Sima Yi, creating a body of advisors around Cao; at the same time he participated in several battles and major events of the era, often giving timely advice to his lord. Cao Cao, in turn, respected Xun Yu greatly and placed great store in his advice."
Xun Yu lived till 212 historically, and on 3K he can very well die on the first turn (190)

Xun Yu

Xun Yu in DW

Cao Ren
Cao Ren starts off the game serving Cao Cao, being
"He played a significant part in assisting Cao Cao in the civil wars leading to the end of the Han dynasty. He was appointed as the Grand Marshal (大司馬) when Cao Pi ascended the throne, and was also credited by the latter for the establishment of Wei"

Cao Ren in DW

Huang Gai
The only reason why Red Cliff's battle was a success was due to Huang Gai's plan, by disguising to join Cao Cao and then ram into Cao Cao's ships with fire ships. Helped Wu greatly

Huang Gai In DW

Pang De
"Cao Cao's general Pang De volunteered to lead a detachment to engage Guan Yu, successfully forcing the latter to retreat several times. On one occasion, Pang De fired an arrow that became embedded in Guan Yu's helm. Since then Pang De was widely known and feared among the enemy as "General White Horse" because of the white steed he rode into battle.
Although Guan Yu could not defeat Pang De in battle, he nevertheless held firm control over the water routes around the area and maintained the encirclement of Fancheng"

Pang De in DW

Yan Liang and Wen Chou
These two are pretty big for Yuan Shao, also for replayability reasons, though these two are the least likely to be added as DLC, if CA did it would be worthwhile to play Yuan Shao
Zhange He
One of the Five Elite Generals of his time serving Wei

Zhange He in DW

Guan Ping
Guan Yu's son, died along with Guan Yu

Guan Ping in DW

Diao Chan
One of the most prettiest women in 3K, also one of the 4th most prettiest women in China

Diao Chan in DW

Xu Shu
One of the earliest Advisor for Liu Bei

Xu Shu in DW

Pang Tong
The Phoneix

Pang Tong in DW

Han Dang

Han Dang in DW

Ding Feng

Ding Feng in DW

Lu Meng
Guy who is responsible for demise of Guan Yu

Lu Meng in DW

Li Dian

Li Dian in DW

Fa Zheng

Fa Zheng in DW

Liu Zhang

Han Sui

10/06/2019 Updated, adding more characters to fix no Legendary Trait, fixed issues in raising other Generic Stats, now only these exclusive characters will have them
Guan Ping, Zhang He, Pang Tong, Xu Shu, Xu You, Xia Jiu, Diao Chan, Yu Jin, Lu Su, also the three starting Yellow Turbans Subordinate with special skills/traits etc
11/06/2019 Added Fa Zheng, Li Dian, Liu Zhang, Han Sui ad fixed others
Updated again, added Wei Yan, Jiang Wei, Da Qiao, Xiao Qiao, Liao Hua, Ma Dai, Guo Jia, Yueying, Guan Yinping, note characters like Wei Yan normally starts Legendary, but sometimes they don't, this simply fix that
Does not require a new game start, can use on your current save
It's very likely that these characters will come as character packs/ DLC in the future updates
Thanks for reading

Put it in
E:\Steam\steamapps\common\Total War THREE KINGDOMS\data