Who am I?

With my hair and beard of black,
I followed in my ancestor's footsteps.
(According to the movies, anyway.)
Who am I?

Caspian?

Nope, but sorta close.

Nikabrik

Yeah, that. 😉

I whispered in his ear,
he shouted in mine,
but year after year,
I kept him blind
to my nightly fears
of an hour's time.
Who am I?

Trumpkin?

Lady of the Green Kirtle?

My guess is Glimfeather, when he brought Eustace and Jill before Trumpkin just after King Caspian had sailed for Terabinthia in the beginning of The Voyage of the Dawn Treader. However, I am not sure of the "nightly fears of an hour's time." Perhaps that could refer to the owl's fears of what would happen when King Caspian died with his son still lost and none allowed to search for him?

Jaygee is correct with the Lady of the Green Kirtle! Because I feel like it, I'll give you a breakdown of the riddle:

"I whispered in his ear, /he shouted in mine," - her sweet lies and croonings against Rilian's nightly cries while bound in the silver chair
"but year after year, /I kept him blind" - her enchantment over him for ten years
"to my nightly fears/ of an hour's time." - in which Rilian "shall call upon you by all that is most dear and most dreadful."

😀

First gloomy,
then joyful,
then frightened,
then relieved,
then panicked,
then gone.
Who am I?

I'm going to say Puddleglum even though my mind's telling me that's not right...

No, but it's the right book. 😉

Golg?

Yes!