Ignia's entire deal is being Natsu's big brother, but he's so much more than a simple sibling rivalry. He's a fire dragon who ate the fire of another dragon, which is wild even by Fairy Tail standards. His backstory is tied up with the Dragon King Festival and this whole mess about wanting to end the era of humans and dragons living separately, basically forcing a massive conflict because he thinks dragons should be dominant again.
What gets me is his twisted sense of care. He sees Natsu as weak for caring about his human friends and wants to 'strengthen' him by making him burn everything that holds him back. It's a really messed-up, almost villainous take on the brotherly love trope. His power comes from devouring the flames of the Fire Dragon God, so he's on a whole different level compared to Natsu early on.
Honestly, he's one of those characters I love to hate. His arrogance is off the charts, but it makes sense given his lineage and power. The backstory isn't super complex in terms of flashbacks, but his motivations and the threat he poses feel substantial.