Yusuke and Kuwabara were indeed having a difficult time with the Makai. They were used to being the top of the small heap of human fighters, Yusuke on top of Kuwabara, but demons, even the lower class ones near the Maze castle, were closer to or just slightly above their usual level of opponents. Taken individually or in small groups, they would have done fine, but demons rarely came singly when they were hunting. Instead, the whole town came out to 'welcome' them, for values of 'welcome' that involved trying to kill them.
It takes a lot for the weather in the Makai to be remarkable. The storm in which Kurama and Hiei arrived was remarkable. This is a good thing for Yusuke and Kuwabara as it caused the demons around them to pause. Of course, there had to be witty repartee as Kurama and Hiei joined the battle. What would a fight be without witty repartee, after all? The inhabitants of the town on the welcome committee were soon a part of history, which is to say all of the inhabitants of the town were dead, made short work of by whip and sword. And Yusuke and Kuwabara helped, too.
Oh, yeah, I forgot to mention, being distracted by other things going on at the time, that Yusuke and Kuwabara had undergone some rather rigorous training by the grandmother you're glad you don't have. Of course, they needed her and she was getting to the point where she was thinking about retiring and maybe seeing the world. She wasn't the rocking chair type. Yusuke and Kuwabara were sure she wasn't the retiring type when she was done with them, either. She tested them in the strangest ways possible, video games, fights in the dark, random things like that. I won't go into too much detail here since you've seen it before and the real point of this story is to give you more of a 'behind the scenes' view of the history rather than retelling the whole thing.
What I will add to what you've seen is that while Yusuke was interested in Kuwabara, Kuwabara suffered the affliction that affects many who are hit with puberty early. While physically more mature than his peers, his level of clue lingered behind others of his age group. Between that and being a bit of a 'romantic,' he didn't see Yusuke as a 'romantic' target. First off, Yusuke could hand Kuwabara his ass, often without breaking a sweat. Kinda kills the romantic, protect you for the rest of my life vibe, doesn't it? So, while Yusuke would have, it never crossed Kuwabara's mind that he should propose anything, despite the fact that Yusuke had made known his interest before, in Kuwabara's dreams.
Now, Kuwabara was interested, after a fashion, in Botan, but she was a bit more forward than he liked, really, so he never pursued her beyond mooning after her when she wasn't around. But, had he been ballsy enough to let her know of his interest, he wouldn't have known what to do with her. Fighting, he knew, romance, though he wanted to believe he was smooth, he didn't know yet. Like a Great Dane puppy, he still hadn't grown into his paws.
Now, let us return to the story's present. The boys and demons made their way to the castle and in. I won't go into too much detail as, I said above, you've seen it. What you do need to know is that Hiei and Kurama were both weighing Yusuke and Kuwabara for their potential as partners, both to fight and to fuck, because, in the mind of demons, that was nearly the same thing.
Well, not Yusuke as much since they'd already deemed him worthy of their attentions, but being sure was never a bad thing and they wanted to show off as much as watch him. Kuwabara was still an unknown and they wanted to sort him out. Kurama could see the signs of 'still growing into himself,' but Hiei wasn't as used to human development. Demons were spared the awkwardness of adolescence. Such fortunate creatures, aren't they?
As you know, Kurama fought first, showing off and defeating Genbu even though he managed to get himself injured, but that was calculated again. Sympathy sex was a wonderful thing, after all, and there was always the possibility he'd get some from Yusuke, after all.
Next, Kuwabara insisted on showing his worth against Byakko, which proved very informative for the demons. Like Yusuke, Kuwabara had an almost innate knowledge of how to fight, even against a rather persistent opponent, one who outclassed him in many areas. Both Hiei and Kurama were favorably impressed and decided that it was indeed worth their time to study him more in depth, though Kurama did caution that their more direct physical attentions might have to wait until the boy's mind caught up with his body and he became fully the man he was growing into.
Hiei's fight with Seiryu was over so quickly it almost drew a moan from Kurama. While usually into more prolonged encounters, Hiei was one of the very few he knew of who could make a quicky as deeply satisfying as a drawn out fuck session and the demon's speed and accuracy were a large part of that.
Hiei and Kuwabara were almost constantly at each other's throats. Part of the problem was Hiei didn't understand why Kuwabara's mind and his body weren't at the same level. While the boy was almost physically mature, his mind lagged behind, not yet adapted to the neurobiological changes his body. Physically, he'd endured the changes of puberty, but his mind was still that of a child. Hiei lived in a world where children grew into adults before their bodies were mature and were thrown away from their family. This overly prolonged and, in his mind, obnoxious childishness in one who was so strong did not sit well with him. And, with Kuwabara rising to every comment he made, the two were bound to have a difficult relationship at best. Kurama endured their bickering silently for the most part.
Yusuke managed to mostly focus on the task at hand, at the need to stop the intrusion into the demon world. Granted, his focus was aided by a small mirror he carried that let him know what was going on and the demons were stupid enough to attack Keiko. While Yusuke's feelings for the girl might not be romantic in nature, he was fiercely loyal to those he claimed as his own and she was his.
Let's pause a moment and consider the mentality of everyone involved in this adventure, shall we? Yusuke's beginning to get the notion that this whole scenario is kind of serious business. Hiei and Kurama know it is because it's been the vaster majority of their lives. Kuwabara, though, believes he's involved in some kind of live action role playing game with super powers and a reset button. But, then, Kuwabara had also thought something along the same lines when it was just him and Yusuke mixing it up, that it was a game to see who would come out on top and it'd never end. Mortality and death hadn't touched him yet, the way it had the others, and he believed he and those around him would live forever. Yusuke knew he could die, had died, and knew what was on the line. Hiei and Kurama fought for their lives, even though demons lived longer than humans in general. This disparity, as it became more obvious to Kurama, made him decide to wait until Kuwabara had accepted reality before making a move on him. If he acted before the boy was ready, he ran the risk of destroying his mind. If he tried to force reality into his perspective, he'd also destroy him. For now, he'd support Kuwabara enough to continue using his strength, and if that meant listening to him and Hiei bicker, then so be it.
The endless stream of zombies and leveling up didn't help dissuade Kuwabara of the idea that this was some grand game, either. The move they used to get Yusuke into Suzaku's tower was definitely one that any excellent RPG would have, a human (and demon) tower to leap over the NPCs that stood in the way to waste energy and HP on. The energy bursts, Hiei's sword skills, Kurama's flower powers all smacked of something Square Enix could use in a Final Fantasy. What was there, really, to prove that this was real life and it could bite him in the ass? Even the ring of demon hounds skewered on his spirit sword around one of the towers was just too 'video game' to be taken seriously.
Even at the end of Yusuke's fight with Suzaku, where Yusuke won, but just barely with his life, didn't serve as a reality check because Kuwabara was able to give Yusuke some of his life force energy and keep him alive, just like Phoenix Down.
So, they returned to the human world, victorious. Yusuke took a few days to recover, during which time, Kuwabara convinced Kurama to go along with his little 'joke' of making Yusuke think that things had gone terribly wrong. After Keiko walked Yusuke home, asserting her claim over him, which he didn't respond favorably to, though he was glad that he didn't get beaten up, Kurama knocked on Yusuke's window, using a tree outside to help him reach it.
"I wanted to apologize again," he said when Yusuke let him in.
Yusuke shook his head. "It's not your fault. Kuwabara likes his games." He sighed, sinking down onto his bed. "You didn't need to apologize again."
Kurama sat next to him, as close as he felt he could get away with. "I also wanted to see you again," he admitted, his voice shy, only looking at Yusuke through the corner of his eye. He knew well how to get exactly what he wanted and he wanted Yusuke and believed the boy was mentally mature enough for the encounter.
Yusuke startled and looked at Kurama again. "See me?" he asked, his voice covering three octaves in the two words.
"Does that bother you?" Kurama asked apologetically. He drew his lip between his teeth. "I…I find myself attracted to you," he added softly.
"But…but…you're a kitsune. Why…I mean, I have nothing to offer," Yusuke stammered.
That the protest had nothing to do with gender was not lost on Kurama. "Kitsune are attracted to powerful people, Yusuke. You are powerful and becoming moreso. Why wouldn't I be attracted to you?"
Yusuke ran his fingers through his hair, rising from the bed and pacing the room. "But…I mean…I've never thought of anyone that way," he muttered.
Kurama stood, placing himself in Yusuke's path. "I don't need an answer now as long as you'll think about it. You have done a lot for me and I find that and your strength attractive."
"You've helped me out, too," Yusuke protested weakly. "We're friends," he added, as though that answered everything.
"I'd like more, Yusuke," Kurama said softly, his hand coming up to caress the boy's cheek. When Yusuke didn't flinch away, Kurama leaned in and placed a light kiss on the boy's lips, lingering long enough to promise more, but not long enough to give him time to panic. "Please, consider more," he whispered before moving away and leaving through the window he'd used to enter.
Yusuke stood, frozen where he was, a long time. He didn't know what to do or how to answer Kurama's request. He'd know from the beginning that something strange drew him to the kitsune, but he didn't know what and that strangeness had grown vastly stronger in the past five minutes.
It never occurred to him to turn down Kurama because they were both male. As he dreamed, he wondered what would happen if he did say yes.