This is specific to my Gajeel and @hasaikiba‘s Rogue, she has permission to use any headcanons revolving around Gajeel and Rogue since we came up with them together. This headcanon revolves around Gajeel and Rogue’s past and the fact they are Kin, see this post, and though they deny/fight it, it’s still there.
Rogue and Gajeel met when Rogue was 16 and Gajeel 20, in the year X783, it happened after when Rogue heard of a Dragon Slayer called 'Black Steel Gajeel' who was one of the top Mages in a guild called Phantom Lord. Travelling to Phantom Lord with the intention of joining the guild, he walked into the building and met the very man whose infamy had drawn him there, introducing himself as Ryos and explaining how he wanted to join Phantom Lord, to which Gajeel responded with a heavy punch and ridicules about how they didn't accept weak children into their guild, Juvia being the only person with enough backbone to actually reprimand Gajeel for it and ask if Rogue was okay.
At the time there had been an age restrictions on guild members, though they were often wavered if the child/teenager held enough magic power and ability to pull their own weight. In order to do this, Jose insisted that each new applicant would need to fight an S-Class Mage to ensure that they were strong enough, if they couldn't they were denied entry and told to not come back until they were sure of their Magic. With Rogue however Jose was instantly interested, having two Dragon Slayers in the guild was an advantage he was more than willing to exploit and so he allowed Rogue to join on the condition that he beat or drew a fight with Gajeel, to which he could not.
Rogue didn't stop coming back and trying though, using every trick in his arsenal to try and beat Gajeel but he couldn't and Jose allowed it because each time, Rogue was a little better than before and he knew that by the end of it, he'd be good enough. Gajeel however hated it, hated Rogue for being as stubborn and pigheaded as he was himself and more than anything else he hated the fact that ever since that first moment when he'd all but punched Rogue out of the guild, he'd felt a compulsion that getting harder and harder to ignore. He knew what it was, Metallicana had explained it all to him before he 'died', but it didn't make it any easier, Gajeel was a man who did not care for anyone but himself and Ryos was not a person he wanted to consider Kin.
Rogue had felt the compulsion too, it was almost brotherly in nature and due to how often that the two were mistaken for brothers it really was no surprise but like Gajeel, Rogue avoided the compulsion. His only need was to get strong enough to enter Phantom Lord and prove to everyone, especially Gajeel that he was not a child and he was strong enough to be taken seriously, he was a real threat. In order to do this, Rogue watched Gajeel, studied his movements in hopes that he could replicate them himself, if he studied the man then he could find a weakness while also getting stronger. To which Gajeel responded with nothing but threats and aggression, wishing he'd just go away.
Eventually though, Fairy Tail came and Rogue's attempts to prove himself would soon come to an end as Phantom Lord was disbanded. Something Rogue had immense trouble dealing with, especially when Gajeel went straight from Phantom Lord to the very guild who was behind it's destruction. He was lost, feeling betrayed by the person his mind had deemed Kin for joining the enemy and he had nowhere to go, meanwhile, Gajeel was avoiding the idea of Rogue entirely, he no longer belonged to Phantom Lord and he no longer had to deal with the annoying teenager who had been a pain in his side ever since he showed up. Not long after, most of Fairy Tail went missing.
After this Rogue met Sting and they, with Frosch and Lector, joined Sabertooth and everyone was seemingly beginning to forget all about the lost Fairies, except Rogue knew that Gajeel at least was alive because the Kinship he felt was still there, it was distant but it was there and though he didn’t like the connection, he knew it would’ve been severed had Gajeel truly been dead.
It had become evident to Sting though that in those years after the guild, Rogue had continued to check up on Natsu of all people, to the point where Sting said Rogue ‘looked up’ to Natsu. This was because Natsu had succeeded in doing what he could not, he beat Gajeel and had also gained him as a friend (considering Fairy Tail was all about nakama, he didn't know Gajeel wanted to destroy Natsu) something that Rogue certainly had never been able to do.
Rogue, now knowing Fairy Tail was back, only had one goal in mind when seeing Gajeel was entering the games, that he had to fight him and he had to win. For all the times that he had put him down, had beaten him, for all the times he had laughed in his face and made him feel worthless, Rogue wanted to show him what he had become and he was determined to beat him. Rogue did not win, he put up a good fight but alas he did not win and it brought back some terrible memories.
Gajeel did not look down at Rogue this time and think of him as weak however and because of this, though he behaved as though he had no idea who Rogue was, he was willing to begin to accept the Kinship that he’d begun feeling all those years ago towards Rogue. There was no longer any aggression or violence towards the other Dragon Slayer, no more than normal at least, and Gajeel was willing to be a nice guy to him.
Rogue however was not easily convinced and so he still remains sceptical of Gajeel despite the obvious change in his personality, he will always remember the man who beat him down every time he challenged him and that will take a long time to get rid of.