Better known as:

The Dead Hand / The Dead King
Orc-Bane / Moon-Bane
The Arch-Necromancer
Dragon Lord / Ash-Walker
The Last Rune
Death

The most famous Blue-Hair in history.

Originally from the eastern (Japanese/Chinese-inspired) land of Rune, Kaede Foster's story often has two sides associated with him.

The villainous side that paints him as the vilest man to walk the earth. His unnatural immortality and use of necromancy, numerous acts of wanton murder throughout history, and sheer apathy towards others. He has terrorized and destroyed entire towns for crossing him. He once waged a personal conflict against the war-loving Orcs and ravaged them so much that their culture transformed into that of honorable nomads. Prior to that, he was among the gathering of eight Blue-Hairs who wished to negate the damage caused by the collision of the two moons above the world, and only he walked away, the rest perished and their souls stolen by him.

The more heroic side, often linked to his habit of taking those less fortunate under his wing, adopting them into his family (hence his last name "Foster"). His later years are dotted with stories of his goodwill, though under the guise of a mercenary. He brought numerous outlaws to justice, stopped plots and attacks against the five kingdoms, and worked closely with the previous Imperial Emperor to maintain peace across Kalv'er. His love for family, adopted or otherwise, was strong. He even adopted a young girl who lost her home to plague as his daughter, raising her with cherished care.

Regardless of which is his true character, several things are certain. Prior to the two decades of darkness that was The Age of Ruin, Foster would assassinate the Emperor he considered a brother. For this crime, Foster's adopted daughter would track him down and use her knowledge to finally kill him, ending the mad immortal once and for all.

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Kaede is a fun character, real cheery, real wholesome.
Jokes aside, my original vision was for him (and all blue-haired characters) to not only share a physical resemblance but a voice also. I'm changing that now, but I'd recommend watching Chapter I and listening to the protagonist, Arthur's, performance. It's not required, and it shouldn't influence anything as they're two separate personalities. But Kaede should sound distinct, older, and wiser.
A little deep, but not a baritone.

The key parts are this; Kaede is blunt, but not emotionless. He has a sense of honor but is not above pragmatism. He cares deeply for his adopted daughter, Hood, and overall has a strong sense of family, being utterly willing to storm hell itself for them. That's a promise he'd keep to the end.

Also, as Kaede is from an Asian-inspired culture (mainly Japanese), I had the idea of him peppering his speech with occasional bits of Japanese, words and phrases as a fleeting habit of his old home. While knowing Japanese is not required, an understanding or even passing knowledge of it would make the additions more authentic (though mispronunciations and struggling with it may also work, as he is slowly forgetting the language because of lack of use).