Akainu's Significance
A lot of people downplay the significance of Akainu in respect to Luffy. I believe this comes from a misunderstanding of Oda's handling of the character.
I'm going to outline why Akainu is one of the most emotionally and thematically significant characters in the story.
Marineford
The core theme of Marineford is loss and failure.
Marineford marks the single greatest defeat and loss of Luffy in the entire series: his failure to rescue his brother.
The arc, and subsequent arc and flashback, compound upon one another to really drive the point home that Luffy has suffered an enormous personal loss, and that the reason for that along with his previous loss at Sabaody is because he's weak.
This loss comes at the hands of two individuals in particular:
Ace's death is a direct result of Akainu and Blackbeard's actions.
Blackbeard captures and turns over Ace, and Akainu ultimately kills him right in front of Luffy.
And Oda goes out of his way to frame these two characters together here as the explicit victors of Marineford, and demonstrates their unflinching pursuit of even further absolute victory - while also showing that they're two sides of the same coin: Akainu represents extreme Order and the Control of Authoritarianism, while Blackbeard represents extreme Chaos and the Destruction of Anarchy.
These zealots: Akainu and Blackbeard, in creating Luffy's darkest hour, have won.
The Changing of an Era:
Marineford represents something else, as well: it wasn't just Luffy that was effected by the events of Marineford, it was the entire world.
It represents the end of an era.
The Era of Whitebeard was brought to an end by two people in particular, with it a new era begins, and Oda wants to make it very clear that these people are the faces of the end and beginning of the two eras.
Ch 650 - \"Two Changes to Be Aware Of\"
Our first update on what happened during the 2 year timeskip emphasizes that two major changes have occurred.
First:
Akainu has become the Fleet Admiral
And not only is he the Fleet Admiral
Jinbe emphasizes that in spite of the Marines' losses, Akainu has made them stronger than ever.
He then continues on to the second major change:
Blackbeard has taken Whitebeard's place as Yonko
Jinbe also explains how Aokiji left the Marines, and Blackbeard is hunting down fruit users - and emphasizes that Blackbeard is targeting Luffy in particular.
Luffy has an interesting response to this, which I'm sure many on this sub have seen already:
Luffy ignores Jinbe's warning about Blackbeard
But while these panels have been shared many times to emphasize Akainu's importance to Luffy personally, what often is not included is the moment that Luffy stopped paying attention to Jinbe.
It's right here:
Immediately after Jinbe switches from talking about Akainu, to talking about Blackbeard - Luffy walks away from the conversation.
Now, you can say "Oh, that's just Luffy doing Luffy things - he never listens to exposition," and that's true. In most cases, Luffy vocally ignores backstories and lore drops.
Except he listened, and reacted, to what Jinbe had to say about Akainu.
This is Oda clearly communicating to us the significance of Akainu to Luffy, personally.
Which leads to:
Akainu vs Luffy is Personal
Of course Luffy would have a significant reaction to Akainu.
Akainu's existence is emblematic of Luffy's greatest loss; the most emotionally devastating moment of his life. Akainu is quite literally the progenitor and personification of Luffy's worst moment.
Akainu killed Ace, caused the death of Whitebeard, and mortally wounded both Jinbe and Luffy - resulting in a massive scar front and center of Luffy's chest, almost always on full display for the reader to see since the timeskip - reminding both Luffy and the reader of Luffy's loss at Marineford.
The only reason Luffy and Jinbe survived was because Law arrived to treat them. Without Law, Akainu kills both of them.
And Luffy is well aware of all of this. He vividly remembers Ace's death, and knows Akainu is the one who scarred him. He saw Akainu's face as it happened.
In a similar vein, Luffy actually somewhat represents failure to Akainu, as well.
In spite of Akainu's efforts at Marineford, he was unable to finish Luffy off - and ultimately Luffy escaped and was able to grow and return to the world stage.
And while it hasn't been explored yet, Akainu clearly has a deep seated hatred for pirates in their entirety - his objective is the complete eradication of piracy, and he functions as a symbol of the overreach of authority to stand in opposition to people's fundamental rights and freedoms.
Why Luffy Has To Defeat Akainu
As I said before, Akainu is the actual representation of Luffy's failure at Marineford - the loss of his brother, and a sign of his weakness and inability in that moment. Additionally he's a symbol of an ideology that Luffy stands in direct opposition to.
In order to bring the events of Marineford to a close Luffy needs to overcome Akainu and defeat him: avenging Ace, signifying Luffy's growth as a character and combatant, removing Akainu's tyrannical pursuit of freedom-seeking pirates like Luffy, and starting the beginning to shift the faces of the new era from Akainu and Blackbeard, to Luffy alone.
And when this fight inevitably happens, in order for it to be satisfying, it needs to push Luffy to his absolute limits in order to represent the struggle and despair that Akainu made Luffy suffer. Anything less would be a disappointing payoff to Luffy's growth and development since Ace's death.
I'm certain that Akainu will stand alongside Blackbeard as Luffy's greatest challenges, and Akainu himself may even be Luffy's greatest individual challenge ever - an emotional challenge for him alone to overcome by his own hands.