The day Sogo met a young werewolf named Tamaki was one he wouldn't forget. It was a memory he kept close to his heart. Sometimes he would think back to that day. It was something he would never change, he didn't regret meeting Tamaki. He became his closest friend and they protected each others. All because of that vow.

. . .

"To think some demons actually think I'm weak. Well, appearances can be deceiving.." Sogo chuckled and he stepped over one of the demons he had just fought.

He had gone into town to buy some food and anything interesting. It was amazing what rare items some merchants were selling. Though sometimes they were just scams. Honestly if you paid close attention to what you were about to buy it'd be obvious. When he had went to buy an item that seemed rare, some other demons showed up and tried to fight him for it. Maybe they thought he seemed weak, something a lot of demons assumed. But in the end they learned he was quite strong. People who knew him knew not to mess with him. Especially when he was angry.

"Now that I think about it though, this item seems like a fake and not worth the price it's being sold at. So you all can have it after all." He smiled and tossed the item to the ground. "Bye bye, try to remember not to mess with me again."

This sort of thing was a reoccurring thing. Fighting others for supposedly genuine items. Luckily he never wasted his money on such things, as he could easily tell if the item was real or not. If it wasn't well, he didn't lose anything from it. But the next fool surely would. Not like they'd get a refund, that word didn't exist to demons. Real or fake, it was all a matter of scamming people out of their money. Wealth and power, both something Sogo didn't care about, even if it was what defined demons and what everyone yearned for.

Being a scientist of sorts was enough for him, testing out skills and making some potions that'd be helpful, depending on the situation. Ones that could heal, poison, etc. All of which could be useful during fights. It was only a matter of finding the ingredients he needed. So that's what he went to go find. Luckily ingredients for making potions were always genuine, mainly because they were sold by demons who made such things themselves. 

It didn't take very long to buy what he needed, unlike rare items and other things, ingredients didn't cost him very much. Not unless they were rare ones as well. Which he barely saw any being sold, unless he got lucky. As he began to pick up the bag of ingredients he bought he heard another commotion. 

'I wonder what's going on this time..' he thought. Out of curiousity he followed the commotion to the source. To his surprise it was another fight, but this time between some demons and a werewolf. Didn't they know not to mess with werewolves? They're lucky it wasn't night or a full moon. Then they would of been doomed. Still, it looked like the werewolf was in some trouble.

Even now Sogo didn't know why he joined the fight and helped fight off the other demons. Supposedly he just felt the urge to help the werewolf and he didn't regret doing so. After the fight he faced the werewolf. Looking at him now that he was up close, no wonder the werewolf was having some trouble fighting on his own, he seemed rather young and inexperience.

"Thanks..for helping I guess.." the werewolf huffed and he sat down.

"You're welcome. Have you never fought before? I've seen plenty of werewolves handle fights like this with ease."

"Hmph, is it obvious? I've never had to fight before, they just attack me out of nowhere too. I didn't have time to react."

"That'd do it. So what's your name werewolf? And how old are you?"

"Tamaki Yotsuba, I'm 1,700 years old. And you?" Tamaki responded with a tilt of his head.

"Sogo Osaka, 2,000 years old. You're still young Tamaki-kun, I assume you go to Evil Academy then? Are you a delinquent or do you skip classes like a so-called real demon does."

"I don't skip them because I care about being a real demon, I just don't like going to class. Not like the teachers even teach the right subject or make any sense. So why bother participating?"

"Consider me a delinquent then, because I actually went to classes. And I'm going to ensure you go to yours, you could learn a thing or two. Regardless if it's the right class and subject or not."

"I just met you and you're getting in my life..! Uu..you're mean So-chan..Ah right, I call people by nicknames, I hope you don't mind.."

Sogo couldn't help but chuckle a little, what a cute nickname the werewolf had given him. He helped Tamaki stand up. "Even though we just met, I feel like protecting you and ensuring you have a good life. I don't know why. Do you have a home at all?"

"I don't..and..I kind of have the same feeling as you. So..how about this? I vow to protect you and you vow to protect me? We can make sure we both have a good life and take care of each others."

Call it strange or whatever you want to, but they both had the urge to protect each others, even though they had just met. So Sogo of course happily agreed to vow to protect Tamaki. That day he made a friend and a protector in a way. As they stayed together and grew closer, Tamaki learned how scary Sogo could be and Sogo learned how carefree the werewolf was. 

Even though they had hardships dealing with each others, being almost complete opposites and all, they made through it all. After all, opposites attract. They made up for each others flaws. So they never regretted their actions of the past, making that vow and staying together.