riddle - Here's a little cryptogram I wrote, you might want to read it letter for letter

Mage's birth seems important. That's all I'm going to say.

J EJPL QW VFIUQT UXWUTH YOSWSUV, HIU Q JQ PIAX PN HSV.

BQIZ BQMKS R QBG WSU MGMTB,

JRE IV E CQC ASWWK JV CLF PNEE QO M EW.

R EN I KMU JRTPTJV, TWVIUQVIT KQESURRH IWH PBQIS BRQFA FIJZM,

WPUNXJUNW J TROF JNMOO YIFZ PCOB,

BSNMCMNMB M XIWX UW UYSS RR UPN FFVCLJK HSOM

Also, if I completely messed up the encryption, don't get too mad....

Oh, yeah. And when you've decrypted that, you've got to solve the riddle. :-)

If anyone (who is bothering with the riddle) is having trouble, maybe read below... It may or may not help.

Not really that important... Now I'm getting all science-y on you!

Answer

I think the answer to the riddle found by Tryth and TheWamts is

a QUARK.

I walk in realms mostly unknown

Realms of subatomic particles.

yet I am part of you.

Quarks are what makes up everything, including your body.

They think I may not exist and am a bit wrong in the heaz.

For a long time after the existence of quarks was postulated, many people didn't believe in them.

If I doz am a bit bipolar,

Up and down quarks.

sometimes charming and other times weird.

Charm and strange quarks.

Sometimes I like being Peer Gynt, sometimes I want to lurk in the Benthic Zone.

Truth and beauty quarks maybe? Or a reference to the word Quark being used in a German play Peer Gynt, according to d'alar'cop.

I got this idea by reading "I am part of you. They think I may not exist" and recalling the following joke:

"Legally speak­ing, how can you kidnap something that is not supposed to exist? I doubt anyone ever accused Murray Gell-Mann of kidnapping a quark, even though he knowingly carried a billion of them around in his pocket." - Minerva Paradizo, Artemis Fowl and the Lost Colony

(quoted from memory - if I got the words slightly wrong, do correct me!) The "sometimes charming and other times weird" was the confirmation I needed for this guess.