There will always be fear.
Unlike the childish beliefs in fairy tales, fear never really leaves. People go through their entire lives with a shadow lurking behind them, fear keeping them in line and keeping them safe. It is not hopes and dreams that stop you from leaping off a building, from giving in to the call of the void. Fear of pain and death keeps one alive, on the thin strip of the present that separates the daredevils and madmen from the reality of tomorrow. Pitch Black knows that, for he is one of a few creatures that see the entire journey of human life.
While they abandon their youthful convictions, Pitch strives on the fuel that the Guardians could never access. Devoid of the powerful force of unquestioned existence children so willfully share, he continues to exist in his dark underground abode. His form changes, nightmares adapt from the frightful black horses with burning eyes into the cold depressing ennui of common existence.
He survives on this thin gruel for some time, regrouping after his latest defeat. No one wants to be not believed in, but he didn't choose this, he would never. Maybe his previous life wasn't heroic, maybe it lacked wonder and all the mushy hopefulness the other Guardians had, but it was his own. His fear saved a group of hunters from chasing a wounded deer into a ravine; his worry stopped them right before firm land changed into a steep fall to their deaths. They praised him afterwards, not all of them, but those with families back in the village who recognized what he just protected them from.
Many hunts went on all throughout the years and Pitch was a lucky charm to the group, his instincts trusted among the men. He was already old when his wits started to abandon him and every crack in the ground, every dusk shadow foretold imminent danger. Fellow men at the bar presented threat whenever they bared their teeth in laughter, for each of them could turn on him in seconds. Mean jokes started to spread and Pitch became a pity cause, taken along for the excursions out of the respect for what he once was.
Resentful and bitter, he pondered leading them into a trap, a cruel revenge for their mindless disconcert. But in the end, with the group seconds from stepping onto a bear's lair, he urged them all to run and stayed behind to distract the furious animal. He remembered the event with unblissfull detail, every claw tearing his skin, teeth in his flesh, fear pouring through his veins, freezing his insides while the vicious attack continued until he could no longer see the light of day.
No one came back for him.
Night came and the moonlight barely reached his remains scattered over the forest undergrowth. But within the pale illumination, he raised again, oblivious to pain and carnage around; he walked ahead through the trees until a clearing let him see the beautiful face of the moon, and it spoke to him. It gave him a name and it let him forget, but he refused, the sheer terror not thirty feet behind the only memory he needed.
If it hurt, it was only for the first couple hundred years; not being perceived for the justful warning, but rather a hateful monster taming people's natural curiosity. They called him the Devil and a thousand worse names whenever they froze in place when seeing an accident instead of foolishly rushing in. Children blamed him, afraid to climb a tree they could fall from; adults called his name in ridicule only to cry in their beds once no one was looking. He watched them from the shadows, from the void on the bottom of a mountain, from the eyes of wild animals, and accepted what they made him to be.
When modern times came, Pitch was thrown away. So many threats overcome, yet so much fear remained inside of the people he so cared about. Guided by science and rationale, he was discarded as a useless remnant of the Dark Ages, helplessly watching one catastrophe after another that could be prevented with his presence. Children were now his last bastion, still personifying immature horror of dark corridors and letting him live inside of it, weak and horrified himself.
Fear, much like humankind, endures. It took all he had to master Sandman's technique, to turn hopelessly happy dreams into a dreading reality of what lies ahead. He made a mistake, and admitted it later, of antagonizing the Guardians, but he could not stand not being believed in, could not withstand the loneliness as well as Jack Frost could. Driven under a wall, he attacked with all he had and lost to a misunderstanding of his very nature.
It took years before he approached them again, with a different strategy and newly-found humility. Not ashamed of the fear that created him, but aware of the balance that had to be maintained. He was ancient, older than any of the commercial symbols the other Guardians became over the years; yet the one he approached was the youngest of them, for his recent memories of being but a background to peoples' lives.
"Long time", he started, and immediately shielded himself from an icy blast.
"What do you want?" Jack kept his distance, wary of his nightmare horses, even though Pitch didn't bring them along.
"Recognition."
It took much of Pitch's resolve not to mold his anxiety into anger, but he kept the searing emotions back and explained as best as he could. The necessity, the progress, how his presence didn't have to be irrational had he been given a place to work.
Jack left without a word, which was better than chasing him back into his abode, but before Pitch withdrew, the boy returned with Tooth, the colorful fairy. She twitched in the air, wary to apprehend Pitch, but eventually passed him the golden teeth containers he was already familiar with.
He barely had to look to recognize the children who needed him. Teeth knocked out by drunken parents, nervous evenings eavesdropping arguments behind the kitchen doors, countless plans of running away from abuse and neglect. The tiny percentage that the happy-go-lucky Guardians couldn't help with all their hope and wonder, children who needed help beyond what their colorful imagination could provide.
Back in the old days, he would encourage them to wield a knife.
How could the Guardians be so shortsighted as to think Santa will make it all right?
They didn't believe to begin with, and so Pitch swore to take care of them. The Guardians didn't trust him, as to be expected, and made Jack supervise his work. Pitch didn't mind; for all the pain he saw throughout the ages, Jack already saw as well. Whenever his frosty joy didn't help, Pitch sent down fear required for the children to take action, to alarm other people, to help them escape abusive homes or avoid getting in the crossfire of parents who simply didn't care.
Not everything is painted eggs and Christmas presents. Many of the rescued children never recovered in their foster homes, growing up with the only idol that ever worked out for them. But some overcame their past enough to believe in Santa, Tooth Fairy, the Easter Bunny and their respective cultural equivalents. It was the most difficult fight that brought tears to the Guardians' eyes regardless of the results.
Pitch Black didn't have to be called a Guardian in order to protect people, as they will always be afraid. He just wanted to talk to someone who understood that fear wasn't evil.
"North doesn't disappear just because children believe in the Tooth Fairy."
He didn't want to be necessary, but he was.
"I want to be seen for what I am, for what I cannot change."
It made sense how they didn't understand. Their perception only lasted as long as a child's twelfth birthday, after all.
"I will guide them away from danger."
Maybe being tolerated was good enough.