If you are caught up or not, what we know right now is that Kirito is in a comatose state. We knew that he was clinging to life and his mind was affected by the poison that killed people in the GGO. That putting him in the Fluctlight machine was the only way to heal his mind while his body recovered from said poison. But right after his fight with Administrator, he was able to contact the real world, able to get a hold of the members of Rath and able to talk to Asuna; it was at that point that those people who invaded cut the power and cut Kirito off from the real world. This also damaged his mind in the Underworld as well, leaving him in a comatose state.
It's established early on that Alice has spent months taking care of Kirito when the two left for her home village after being persecuted by the other Integrity Knights at the tower. "Why does she defend him?" "Didn't he kill the Administrator?" After all those months, the Dark World finally begins to invade and Alice is forced back into action but refuses to leave Kirito behind. It is at this point we find out just what is wrong with him. That his fighting spirit and will are crushed, but his determination lives on, it was shown when Integrity Knight Commander Bercuoli used some kind of power to attack him but his determination stopped the attack that would've cut him if he let it hit him.
Basically, Kirito was inside a shell but he was fine.
That's when people he knew started to show up more, with the onset of war against the Dark Territory and Alice refusing to leave Kirito's side she has Kirito and Eugeo's former pages Tiese and Ronye take care of him. They tell Alice they're to blame, that the only reason Kirito is like this is because he fought the Administrator when they were both taken away after saving the girls and thus violating the Taboo Index. Alice reassures them that wasn't the case and it was always part of their plan, to stop her and save Alice. Upon their reunion you see Kirito more at ease.
Shortly after we see the return of Asuna, who has logged in as the goddess Stacia account, giving her high privileges and thus the reunion between her and Kirito (after she gets in a fight with Alice who has clearly fallen for him in typical SAO fashion). But hearing her voice he perks up, there's motion, then she hugs him and he cries. He realizing who's there finally, it's put him at ease and coaxed a little bit out of him. Alice being alive and not dying by the Administrator's hand, that she kept him safe because he kept her safe kept Kirito in "good spirits" and kept him "healthy" as he could be. That combined with both Ronye and Tiese's return. His mentor at the academy Sortiliena's return (who Kirito obviously had feelings for despite his relationship with Asuna). And of course the massive reunion with Asuna. All of these left Kirito looking more "alive" than ever and I think that's part of what's going to bring him back.
Not the majority reason, I think that will have to do with something else major. But seeing as Asuna is there, the other girls, Sinon has arrived and Leafa shortly to follow, then with their talk: Lisbeth, Silica, Klien, and Agil for sure will show up in the Underworld. With a heavy implication that other major ALO figures like Sakuya, Alicia, General Eugene, and the Sleeping Knights faction at the very minimum with potential for more ALO players to convert their accounts. I do believe that having people around him, familiar faces, will be half of the reason that Kirito comes back around.
That's the thing, Kirito watched his best friend die in a fight that they were both in, and he blames himself for it. The same way he still blames himself for Sachi's death in the original SAO. She was Kirito's first love, it's heavily implied, and I still think based on my older article that Asuna is just a place holder because she reminds him of her. However, when he let Sachi die he was devastated and she was the only other person that Kirito has let die, he even references how he refuses to let party members die when he fought with Leafa once.
It's going to take a connection to Eugeo whether it's through the Blue Rose sword "talking" to him or Kirito having some kind of near death experience that allows him to communicate with Eugeo. Something that will allow Eugeo to give him the, "it wasn't your fault, you helped me through everything, friends to the end, I know you've got this, now go help your friends and beat the bad guys" speech.
After that speech I think we will see the return of battle-ready Kirito. I also think through some magic, probably some Goddess magic that the Blue Rose sword will get fixed; as well as using sacred magic to heal his missing arm allowing Kirito to both be "Dual-Wield" Kirito again and also to have both his sword but a piece of Eugeo with him. A piece of Eugeo that I think that upon the Release Recollection of that sword and Kirito's sword combined is what we will see kill Emperor Vecta.
It is Eugeo first and foremost followed by his friends that will see the return of Kirito.