Whoopie! Our main hero! He knows Kuwabara because he's been rivals with him for quite some time. Of course, Yusuke always wins, but Kuwabara is determined to defeat him in combat one day. Yusuke is the Spirit Detective, and is the powerhouse of the four main characters. He died in a car accident by risking his life for a kid, came back to life for his good deed, and now kicks demon ass for a living. He's a delinquint who trains under Genkai and inherits the Spirit Wave Orb from her, which allowed to become much stronger than before. After about episode 21, Yusuke and Kuwabara start becoming best friends, and they both care deeply about each other, even though it may seem gross to them. But boys will be boys! ^_^

Ups
Of course we all know that Kuwabara and Yusuke have some sort of relationship together, but it's definitely not a romantic one. *is killed by diehard fans* B-but who says they can't be together?! *is killed by diehard YusukexKeiko fans* ... Just read below...

Downs
We must admit, that both Yusuke and Kuwabara don't seem the type to start liking males, let alone each other, but we can't be ignorant. Let's just state a few downfalls. First, the most obvious ones:

Overall
I love this couple. They would be a really cute pairing, as I am a die hard fan of them. I think they could definetlely go together, if they were gay, but unfortunately, they never show signs. I bet if they did come out of their shells though, they would take it slow and eventually admit their feelings for one another. Too bad Keiko and Yukina are blocking that part of their minds.

Kurama

Kurama; regular human in daylight, Spirit Detective in moonlight...or whenever Koenma summons him. Yeah, I just killed the dramatic tone there, but let's get to the point. Kurama used to be an enchanted spirit fox called a Youko in Japanese Mythology. He used to steal treasures as a hobby until one day he was shot by a powerful hunter and had to escape to the Human Realm in his spirit form.
He posssed the embryo of an unborn baby that had not yet aqquired a soul. He was soon born out of Shiori Minamino, and became known as Shuichi. At the age of ten, he regained his spiritual powers, but stayed with his human mother because she grew sick.
About five years later, Hiei came to him with an offer. He would help Hiei and Gouki break into Spirit World and steal the Dark Treasures and he would be able to save his dying mother. Kurama relunctantly agreed, and later was reformed by Yusuke Urameshi. After that, he became part of the Spirit Detectives.

Ups
KuramaxKuwabara, huh? Kinda like Beauty and the Beast, eh?! XD Just kidding, but seriously, kudos to the person who thought this couple up! Let's look at some evidence:

Downs
Things aren't looking that bright for these two, huh? :(

Overall
I really think this relationship is more of the friendship/teacher & student type. They become better friends near the end of the show, but not enough so that they would be in love with one another. This is probably what we call 'a no evidence couple' such as HieixBotan (Don't care if you flame me, but I hate this couple), YukinaxKurama, or KeikoxTouya. Couples that wouldn't make sense what-so-ever, basically. ^^

Hiei

Hiei is a demon with a dark past. He is the user of an amplifyer called the Jagan, which means Evil Eye. He's also the master of the Dragon of the Darkness Flame technique, which is a very powerful and evil attack, and he's a swordsmaster. Hiei was the result of a forbidden breeding between an Ice Koorime and a Fire Demon. His mother resided in the Island of Ice Koorimes, and when she gave birth to Hiei and his twin sister Yukina, the elders greatly disapproved of the presensce of a male, let alone a fire demon.
He was sentenced to death, and they would throw the beast off their island. Before they did throw him off, a best friend of Hiei's mother stuffed his mother's teargem necklace into Hiei's bundle of bandaged talismans. They threw him off the Island as the 'Forbidden Child'.
A band of thieves found Hiei, and raised him as a theif. Soon, Hiei grew to be too strong for the clan, and they soon abondanded him as well. For the second time, Hiei was alone. He decided that he would revisit his homeland for revenge, and began to look for the island. Later, he lost his precious teargem in the wilderness of the Makai and set out to find Shigure, the implanter of the Jagan, so that he may aqquire it, and locate his lost treasure.
After receiving the eye, Hiei was reduced to the power of an E class demon. He trained with Shigure and quickly found the Island of Ice Koorimes. After finding his mother had committed suicide from the loss of her son, Hiei left the island, but met the woman who had thrown him off. She told him that Hina, his mother, gave birth to two. A female and a male (him).
The female (Yukina) was spared because she was born female and with the blood of an Ice Demon. Hiei set off to find her and eventually came across the Human Realm. There, he met Kurama and fought with him to defeat an evil demon terrorising Kurama's hometown. He soon set off again, and later returned to Kurama with a plan to break into Spirit World vaults. Fortunately, Yusuke stops his crazy idea of taking over the world, and he comes back to his senses.
He rejoins the anime as part of the Spirit Detectives, and after that, he grows to master the Darkness Flame technique and gets to be the second strongest in the team. Hiei's past is not only the most mysterious in the show, but also the saddest.

Ups
HieixKuwabara? Whoever came up with them, brilliant. Brilliant, I say!!! Anyway, let's look at some (wavering!) evidence:

Downs
We all knew this was coming...oi!! ^^;

Overall
Uhhh, not to sound weird or anything, but this is my favorite couple. Once again, this is a 'no evidence couple' since they practically have nothing in common, other than they care for Yukina. I'm guessing based off what we know, they can't be a couple, let alone good friends. Still, despite their differences, I think they'd be cute together. ^^