This is from the viewpoint of one Alexander based on Regression.
Regression
"Too much of the past for one to memorize"
- Otto has lived 500+ years of life. He's merely a shell clinging to life.
"Too many words remained for one to read through the lines"
- Too many good memories and past occurrence with regrets forever etched in him. He isn't able to understand what may have been for its been too long.
"The ebb and flow of the crowd floods the world and paradise
Along the path of time"
- From the moment Kallen was sentenced to death by the people's decision, the regression of humanity (and himself) was ingrained over the course of Otto's lifespan.
"Every night brings a dream but the day, relentlessly keeps me awake"
- He is experiencing living hell day by day. Though there is still one meaningful goal and dream to cherish in life, that is to say, Kallen. The only thing, only person keeping him going.
"All the rest will be torn up whenever a choice is made"
- Sacrifice. This has always been Otto's methodology. Sacrifice anything for the greater good. Summoning a Honkai beast to destroy everything in order to save Kallen. Sacrifice Cecilia (Sirin battle) & all other Valkyries. Sacrifice even the world (Kolosten arc).
"Every living soul in the fray striving for their own safe place"
- Everyone is fighting for what they believe in. Otto himself, Kiana, Durandal, etc.. Everyone is defending their hopes and ideals. Their dreams. Their homes...
"Life is too long to end at a grave"
- He understands that a sinner like him no longer deserves a memorial in the world. He has too many sins, too many mistakes that cannot be undone. He has regressed to the point of no return. He even mentioned that he hopes everything would start anew when it is passed down to Theresa.
"Just a drop of water suffices"
- Merely trying to revive/save 1 girl over the course of 500 years of Humanity.
"Encompassed and swallowed through space by the universe"
- Toyed with by fate like a clown no matter the timeline and method. Even in the bubble universes where he didn't go down the spiral of Clown Alpha, he still suffered.
"Back to the source
Gone are those years living for a reason
Here it comes the moment of the scene of lost and found"
- Countless attempts to revive Kallen but barely any progress repeatedly through the course of his life even though he ended up fully devoting and dedicating his entire life for this purpose, it was barely attainable.
"Personas played out on the stage
Will return to the self when there's a curtain call"
- No matter how things played out, it'll eventually lead to the pre-determined Destiny you're born to traverse through and, to end with. Also a reference to Sixth Serenade and her fate. Cause y'know, phantom thief.
"Every night brings a dream but the day, relentlessly keeps me awake"
- Currently fighting to death for the "Dream" after he witnessed the "Miraculous" Kiana K423.
"All the rest will be torn up whenever a choice is made"
- His choice is made. His life for Kallen's life. His legacy would soon be over. A timeline sprouts anew on the other side.
"Everyone has their own desire leading to the ultimate"
- He's now heading towards his ultimate goal. The climax of his life. One he has desired for 500 years. He also believes that the "heroes" (Kiana/Durandal) and everyone else will one day reach their ultimate goal as well (as seen from his farewell ghostly speech to them in-game).
"Life is too long to end at a grave"
- Scenes reflecting all the actions and sins of his past. He personally accepted that some of his actions are irredeemable and hopes his sins and ways of the past would be forgotten and cleansed by Theresa's goodwill and leadership eventually.
"Just a drop of water suffices"
- Just wishes to spend just another moment with Kallen once more. He wishes to change the fate of Kallen Kaslana, even if it is just another distant timeline where he is no longer there.
"Still I wish to embrace the world with my thoughts"
- Otto Apocalypse, the "Inventor" that was understood and approved by Kallen. The recognition that then led him to strive to become the greatest inventor to assist Kallen and eventually led humanity against Honkai. Over the course of time, the methods slowly regressed and started to leave a bitter taste.
"A eulogy"
- Despite all the sins, no one can deny that Otto Apocalypse has left a legacy worthy of praise in several aspects. Without Otto, Humanity may not stand against Honkai as it does now. There would be no Kiana. Schicksal would not be as it is today. We wouldn't be able to criticize him since he'd be gone.
"Time to leave where I have stood so long"
- 500+ years of legacy and humanity's progress.
"Letting you go recover traces overlapped"
- I leave the rest to you (to Theresa in the present, and to Kallen in the new timeline).
"Ends, then Begins".
- My story ends here. Your story starts here. Live... Kallen.
Otto Apocalypse. Clown Alpha (Clown α). Raksha. Overseer. The foolish Inventor.
You were a truly despicable man. You lied and you harmed. You created and you destroyed. You even once pulled a Ship of Theseus experience on one of your Previous S rank Valkyries.
You were... You are, one of the most disgusting, irredeemable bastards I have ever met or known.
...
But. You also did so much good for the world. You helped Durandal grow to become who she was. You developed a cure for Honkai, or at least a means of mitigating most of it. You gave Theresa, a mere clone of Kallen and Vishnu, a new lease on life. You made sure the world had a means of protecting itself even if that was only an avoidable circumstance.
I want nothing more than to tell you I hate you. But I don't. Not anymore. You are a hypocritical existence. A flawed broken man. But you have earned my respect. And for that, I wish you the best in whatever afterlife you find yourself in.
Farewell Otto Apocalypse. Your tale begin from a small ember of inspiration and admiration, and it ends with a Blazing Bonfire of passion.
Farewell, the Beloved Inventor of Kallen Kaslana.