Stardust Railway: Lumen

Single female protagonist, mirror-style (enter at your own risk if you mind), semi-slice-of-life, invincible in the second half.

It's sweet, please believe me (sincere face)

Also, please note the pairing of Ying Xing and Bai Heng (if you mind, please don't read this).

...

Monsters that live in the abyss will relentlessly pursue light once they have been embraced by it.

He spent the first half of his life in the abyss, but was pulled out of it by the girl who mistakenly entered his life.

"Hey, do you remember? The first time we met, you wanted to kill me."

"……I was wrong."

"There's no need to be so decisive."

"I apologized not because I was afraid my wife would beat me, but because I was afraid she would run away."

...

I was born in the abyss, and from now on, I will always be by your side.

The abyss is the first half of my life, and you are the second half.

My later life will be bright, not because I've left the abyss, but because you're by my side.

Don't you have a name?

"No."

"Does it have a name now?"

"Um."

"From now on, let's call him Yuanming."

...

The plot may be altered; please do not enter if you mind.

The main character and the female lead will later travel back to the world of Honkai Impact 3rd. If you mind this, please do not read.

Some settings haven't been officially released yet; I'm relying entirely on my own analysis and speculation (please don't betray me, thank you).