Isekai love game

A thousand years ago, a great catastrophe befell the world; the sky cracked open, releasing magical power. A dangerous creature emerged with it, and other magical creatures were created, granting powers to humans. These creatures came to be called monsters. Not all monsters are evil; many don't even possess enough power to harm a human, and most appear in nature and spend their entire lives without ever seeing a human. There are 10 levels of monsters, corresponding to their levels of magical power, which is why high-level monsters are rare. Humans who gained magical powers and use them to defend humanity from monsters are called hunters. The first hunter, who killed the first monster king, was the precursor to the Tohp family.
An ancient prophecy says that in a thousand years a king of monsters will arise and yet another great calamity will befall the world.
Ten years ago, a level 9 monster massacred the Toph family after being imprisoned by the former patriarch. Only Myo, the new head of the family, and Gael, her adopted brother who hides the fact that he is a half-monster, survived.
A young hunter named Yuma gained renown in the world and ended up joining the Toph siblings on the secret orders of their master. The three travel the world hunting monsters. Yuma ends up falling in love with Myo, but he can't escape the fact that he's with them on his master's orders to observe a map the Toph siblings possess—the map of valleys and destinies—which shows where every monster in the world is located.
One day, a marquess hires them to hunt a monster. The marquess's daughter, Lady Fay, falls in love with Yuma, but knowing he loves another, she pretends to be Myo's friend while plotting countless schemes to separate her and Yuma. Gael discovers this and marries Fay to get revenge for her hurting and mistreating his sister. Yuma and Myo also marry, but she discovers the truth about Yuma, they fight, and discover that the monster king is inside Myo. The world king kills them and then destroys the world.

Yuma - male protagonist; kind and responsible to everyone; renowned and strong monster hunter; hides a guilt-ridden secret from his past.

Myo - female protagonist; fair and upright; head of the Toph family; wants revenge on the monster king who disappeared after the massacre.

Gael - secondary male character; Myo's younger adopted brother. He hides from everyone that he is a half-monster. Stubborn, distrustful, arrogant. Myo is the only thing that matters to him.

Fey - female secondary character; daughter of a marquess; spoiled by her father who gives her everything she wants; egocentric. She is in love with Yuma and pretends to be Myo's friend, but behind the scenes she plots a thousand and one things to separate the two.