All Might!
"Have no fear citizens of this alternate universe... because I AM HERE!"
"When Batman transported me to his universe and explained to me what was happening, as much as I wanted to help him defeat Brainiac, I told him that I was on the way to save young Bakugo from the League of Villains and that I needed to be transported back at once! When he argued that Brainiac was much more powerful and smarter than All For One, I argued that I wasn't at full power and my powers are on a time limit so once three hours were up, I'll revert to a weakened state, he said that he and the Insurgency will by me just enough to use my powers again once they do run out."
But for now, Brainiac has to be stopped.
To Batman's credit, he was right about Brainiac being ten times stronger and more intelligent than All For One.
But despite him having an advantage or two, there was no need to worry as the Symbol of Peace managed to put a stop to that dastardly collector of worlds.
Though it came at a heavy price... I use the last ounces of my Quirk known as One for All to knock Brainiac out.
Once Brainiac was in the Insurgency's custody, that's when things got worse as the hero now turned villain named Superman and his Regime of "heroic" murderers tried to use Brainiac's technology to control Earth in their misguided way of justice.
However despite me using my Quirk all up and despite me being in my weakened state, I couldn't just stand by and evil triumph. So, I stepped forward in my weakened state.
Before Superman could attack me, that's when something really strange happened.
A strange, blue ring crashed through the ceiling and landed on the index finger of my right hand.
Just then, a voice spoke to me from seemingly out of nowhere as time seemed to slow down for some reason. Probably for dramatic effect for things like this.
"Torshinori Yagi, you have the hope in you to never give up. Recite the oath and you will bring hope to the rest of the universe."
"I'll be honest, it felt rather weird to use what was essentially a support item for power instead of my Quirk, but given the current situation, I had no choice.
So without hesitation, I recited the oath of the Blue Lantern Corp.
"With strong hearts full, our souls ignite, When all seems lost in the War of Light, Look to the stars, For hope burns bright!"
When I did, a strange, blue light washed over me as it gave me a whole new costume.
Despite me being relatively new at using a power ring, and also with the help of Batman and his friends, including the Green Lantern, who sort of became my mentor during the fight in showing me how to use the Ring of Hope, we managed to detain Superman and his band of maniacs in the Phantom Zone.
In gratitude for my assistance, Batman and the Insurgency offered their help in rescuing young Bakugo from Shigaraki and the League of Villians. As much as I wanted to declined the offer, I have seen what they were capable of. So I accepted the offer.
Who knew that someone who had no Quirk could be a hero.
Maybe young Midoriya was onto something after all when he once asked if me if someone could be a hero even without a Quirk. At the time, I told him that there was no way that a Quirkless person couldn't be a hero. However, if this experience taught me anything, it taught me just how wrong I was in that regard.
And so with the Justice League's assistance, we managed to get back to my universe and save young Bakugo as well as the students who defied orders to rescue him themselves as well as defeat All For One.
After arresting All For One, I told the press and the other heroes who were demanding answers what happened. Despite me no longer having a Quirk now using the Ring of Hope, I was still the Symbol of Peace.
I even kept the ring though I only chose to use it for emergencies.
After all, I still had to train young Midoriya to be my successor as the next Symbol of Peace after all.