Elrond is not a the center of anything ? maybe elrond counsil, well, ok, but noldor and sylvan brothers "in cards" not in lore.
Third is about lore, not about cards. Don't know what is black and white for Elrond here...
Fourth... well, not really. Healing is not when losing hope and not about a shield...
I would like another hint. A bit clueless here.
Yup !
Somebody didn't liked lack of reasoning on my previous right guess, so I'll break this one down for you:
"I'm the watcher at the center" -Gatekeeper. Also, geographical position of Orthanc.
"I'm a man with Sylvan and Noldor brothers" -Came out in a deluxe expansion where Noldor and Silvan traits started to get explored further.
"I work for black and white alike" -Saruman is White, but his soul is Black. Or Black is a Sauron, for whom Saruman works.
"When you lose hope, I'll be your shield" -Readies when Doomed card is played, best for defense.
"at the center" is also because orthanc is in the center of isengard.
"brothers" refered to Watcher of the Bruinen and the new Sylvan Ally that shares the common ability to be able to block more than once.
"black" was for the tower of Orthanc
Your feedback is very instructive in how riddle are hard to make and how I should make them, thanks.
I'll take the thing you don't need, but I don't promise anything useful in return.
"New players love me, advanced players hate me" (i might be subjective ^^)
"I can give you big happy moments, or big fail moments"
"I am ok with each sphere, but not with almost all cards"
Leadershit Aragorn.
Eleanor?
leadershit asterix hirluin
I'll take the thing you don't need, but I don't promise anything useful in return.
Btw, why no one tries to guess this? 2 hard?
you didn't quote, we didn't see it was a riddle. 
also very short...
Bifur ?
Nope. I'm afraid it'll get too easy if I give something else.
It can be very good tale but well equiped fit better.
Ok. A slight clue. It's a character.
I'll take the thing you don't need, but I don't promise anything useful in return.
Btw, why no one tries to guess this? 2 hard?
Ken-eyed Took
"New players love me, advanced players hate me" (i might be subjective ^^)
"I can give you big happy moments, or big fail moments"
"I am ok with each sphere, but not with almost all cards"
Wow, i thought you find out at the first try. I dont know if displays another clue
@Mndela : Hidden cache ? I don't see much cards that are love by new players and hated by advanced players.
It might be a little subjective from my perspective, but it is true i think. Usually when advanced players have experimented nasty fails moments from it cause of bad movements of bad players.
Not took.
Erestor?
Erestor!
"New players love me, advanced players hate me" (i might be subjective ^^)
"I can give you big happy moments, or big fail moments"
"I am ok with each sphere, but not with almost all cards"
Palantir?
No, Palantir isnt.
I guessed someone could answer so. It is not Palantir. Palantir can be attached to any hero you want, so 2nd part of 3rd text doesn't resolve.
Edited by Mndela