OC x Nico Robin and Nami Fanfic: How to develop the characters well?

The most powerful demon of Hell reduced to a mere slug! Thanks to his unique ability to absorb abilities of the creatures he ate, Voren became the strongest demon in Hell. But just when he was ready to relax and enjoy the luxuries he conquered, gods descended from Heaven to take him down. One against many, Voren fought valiantly, but in the end, he was defeated. Still, the gods couldn’t break his spirit. He lost all his demonic powers, but even all of gods’ efforts couldn’t wipe out Voren’s memories—and his unique ability. As a last attempt to destroy him, gods made Voren reincarnate as one of the lowest creatures there is—a slug. But even that won’t stop Voren from taking his immortality back and getting his revenge. He will reach Heaven… through violence. And enjoy everything the mortal realm can offer on his way there. Warning: contains bloody violence, crimes against animals, crimes against humans and what else you can expect from an evil protagonist. (please, read the prologue)

After Frisk finally managed to bring peace to her universe a certain glitchy skeleton appeared out of nowhere and erased it from existence, but luckily the goddesses Fate and Destiny found her Soul and decided to let her reincarnate, what will Frisk do? And will she be able to stay a pacifist... (I am not a native english speaker, so please forgive me for any spelling mistakes, this is also the first novel I am writing, so don't expect high quality stuff, this is just going to be some half-assed story, that I am writing because of quarantine, and I repeat DON'T EXPECT QUALITY! Btw obvious DISCLAIMIER, I don't own BNHA or Undertale and the cover art was made by Patzzi and reedited by Solunary.)

Synopsis.

The story of the son of a Kryptonian and Tamaranian, outcast and abandoned by both races.

Niall was the average male, but with an added passion for Sim Dating RPGs. One drunken night, while chatting with his friends, Niall finds himself in an alley with a blade protruding from his waist. --- -Welcome to Hero Chronicle Role: Villain Objective: Make this story yours. --You have been granted the Sim Breaker, enjoy Young Miss. Cover art is not mine. Will be removed if requested by artist. We have a discord server now: https://discord.gg/8vSfAD9yCj Feel free to join! Crowe#4408 if you're having difficulties joining! This version of the novel has been cancelled. Please look forward to the revamp.

¿Qué haces cuando te despiertas y te encuentras dentro del juego que amas? ¿Qué haces cuando te das cuenta de que no solo te has convertido en un personaje, sino que incluso has regresado en el tiempo antes de que el juego se haya lanzado? ¿Qué sucederá cuando las dos realidades de nuestro protagonista coincidan? Han Xiao era un jugador profesional antes de su transmigración. Usando los conocimientos de su vida pasada, Han Xiao recorrerá el universo mientras se prepara para la llegada de los jugadores. Esta definitivamente no es una típica novela de transmigración.