Raiden is Sho's younger sibling and best friend for most of his time in SAO and later in the real world. Very similar in some aspects, Raiden and Sho really bonded when they played video games together. Unlike Kuzon, the youngest and most social of the Shin brothers, Raiden and Sho would often skip social outings just to play the latest PC or station game. They always played games together. MMORPGs, FPSs, RTSs, and every genre in between. The two brothers created the ultimate team. One was the tank and the other was the damage dealer. That was their usual combo. Sometimes they would switch it up, both being heavy damage dealers. In every game they played, they made a big splash in the game's community. Everyone knew the Shin brothers by their usual usernames, "Storm" and "Scropio". They were pretty famous, but the limelight was placed more on Raiden. Luckily for the two, this didn't put a damper on their relationship. Instead, it seemed to increase it. Sho become much more protective and caring to his younger sibling. They now had something in common. Together, they bonded through video games and the virtual world.
When SAO started, Sho become even more protective towards Raiden. He was almost always around his brother. The two never separated for more than a few hours. Even if they did, Sho always kept an eye on his brother's health and location. If Raiden looked like he was in a bad position (and to Sho that meant at 80% health), Sho would immediately break off and leave whoever he was with. In the death game, he was the most overprotective brother ever. He did everything to ensure his brother's safety, no matter what the resulting cost was. It didn't matter if he got hurt or if others got hurt. As long as Raiden was okay, Sho was fine. That was the bottom line of it. Raiden had to live. He had to live or Sho would blame it on himself.
Aside from the protection that Sho places on Raiden, he also benefits from the relationship. Due to his usual evasive nature, Sho almost never would help anybody in need. His morality or maybe lack thereof, would have him running for the hills at any sign of trouble. Why then, has Sho not run off already? The reason is Raiden. Raiden reins in Sho and provides him with the morals that he sometimes discards. He keeps his brother in line and in check. If it wasn't for Raiden, Sho would have never even gotten anywhere with friends or any kind of relationship. Thus, Raiden is the backup for his brother. Sure its a mutual back and forth kind of thing, but it is mainly Raiden who is the backbone to their relationship.
The first time that Sho had laid eyes on Iroha, his first thought did not go to her. Instead, it went to the pie sitting in front of her. Immediately, he tried to buy her pie, but unfortunately it was not for sale. Not one to give up on an obviously aromatic creation, Sho swiped the pie, taking it in his hand for his own. Being shy, Iroha could only sputter and hope that Sho would find it in his heart to give her the pie.
Well, unluckily for her, Sho was not very warm and cuddly back on floor 3.
Seconds before he was about to make off with the apple creation, Seya stopped him with a raised hand and demanded its return. Sho reacted quickly and dematerialized the item, placing it in his inventory. He was going to walk off when he caught a sight of Iroha, sniffling and looking quite weak. For some strange, odd reason, Sho grudgingly offered to share the pie. Since it wasn't his pie and he had no authority over it, it made no sense. Iroha wasn't about to share something that she had worked so hard on. However, being extremely kind, she allowed him one bite. Smiling slightly, Sho declined, walking away and disappearing into the crowd.
This was the beginning of Sho's attraction to Iroha. It was a strange attraction, an extremely weird pull. After his first interaction with Iroha, Sho could not take his mind off her. She was just there, swirling in the darkness and forcing him into a very agitated state. He paced, he walked, he ran, he fought, he trained. Nothing worked. She was just there and would not leave. He discussed it with Raiden and Saito, and they both laughed, chuckling over Sho's erratic state. This was not the Sho they knew. He didn't walk away or banish the thoughts from his head. He was basically confused over the entire ordeal and he had no idea how to deal with it.
It wasn't because she was pretty, even though she was. It wasn't because she's gentle and kind, even though she was. It wasn't because her smile somehow made his heart run, even though it did. It wasn't because she was kind either, allowing him a bite to eat even though he shouldn't have gotten it. It was how she reacted that got him. For some reason, her scared expression somehow stuck in his mind. The morals and ideals that Sho had drilled into him came surging back, throwing him into discombobulation.
So going against his entire nature, Sho went back to apologize. After finishing, he turned to leave and walk off. Seya, scheming a hook up between Iroha and him, somehow got Sho to join her and Iroha at dinner. One thing led to another and Sho's vigorously eating Iroha's delicious food. He showed an intense lack of manners and other impolite things, grossing out the very ladylike Iroha. He did not make a good impression.
Then came the wild card.
Iroha pulled out a bottle of red wine. She said she had been saving it for a special occasion and that day was the day. Sho, not one to drink any alcohol, just sat and watched Iroha down glass after glass. She didn't really have a tolerance even if the wine was fake. She managed to get herself intoxicated