Hey Fairy Tail
We all know Natsu loves to fight and he has several rivalries. Natsu/Gajeel is one. That's was the first battle between dragon slayers. They even fought against each other while battling the twin dragon slayers. Natsu/Laxus is another one that is totally one sided. Laxus beast the crap out of Natsu. In the battle of Fairy Tail it took Natsu, Gajeel, Erza, and Mystogan just to tie with Laxus. Which makes me wonder what a battle of Laxus versus Gilldarts would be like. Speaking of Gilldarts, Natsu/Gilldarts is another rival but more of a teacher/student relationship. Natsu/Erza is the fourth, while Natsu strives to beat Erza this rivalry is more like a sibling rivalry. The last one and the one that has the most history is Natsu/Gray. These guys have been fighting since they joined Fairy Tail. Where as though Natsu seems to have the power Gray seems to have the fight. Most of there flash backs as kids has Gray beating Natsu but not by much. While Natsu just likes to fight Gray seems to have a need to test his strength against Natsu. In the Grand Master Games story arc they both had a moment when they thought of each other saying they would survive to fight each other again. At the beginning of the Sun village story arc they went on a job together and fought for three days. Even sucker punching Erza. They have always been linked together through battle.
(Disclaimer, Stop reading here if you have not read the Manga)
Now we all know what the acronym E.N.D stands for and Gray now knows how to use devil slayer magic. Gray has also vowed to get revenge for his father and to kill E.N.D. It seems there will be a battle between them that will test there friendship. I don't know if this will be before or after Natsu's fight with Zeref but it is destined to happen. This might be the best and worst time in the Fairy Tail series if Gray decides killing is more important to him then his friendship with Natsu.