Yui/Plot

Guild Founding Arc
While Mai argues with Yui over if they should restart their characters, Maple invites them to join Maple Tree and decide to accept the offer after discussing it with her and Sally. After they joined, Maple and Sally soloed a layer boss each and brought the twins to meet the other members on the 3rd layer.

Mai and Yui went grinding with Maple at the Labyrinth of the Poison Dragon to level up. They wore helmets and rode Maple's demon form to prevent their information from leaking before the 4th event. Mai and Yui learned the [Conqueror] and [Destroyer] skills while having fun rushing the dungeon.

Another day, Mai and Yui learned how to evade wind-up attacks (i.e., penetrating attacks) from Sally.

Guild Competition Arc
During the 4th event, Mai and Yui defended Maple Tree's base for most of the event. Their high attack crushed countless invaders in one hit.

After Maple rushes to save Sally, the twins let their guard down after hitting Dred and die.

As a change of pace, the twins left the base with Maple to attack other guilds. Many guilds were raided, and the twin's attack power made quick work of other players. The Kingdom of the Flame Emperor was lucky because Mii's magic disrupted Maple and the twin's teamwork.

During the raid made by the Congregation of the Holy Swords, Mai and Yui were liabilities that were protected by Chrome. Near the end of the assault, they killed Frederica through her [Multi-Barrier].

Rulership Arc
Mai and Yui went with Maple to hunt ghost monsters for their quest. When Maple's [Loving Sacrifice] skill was sealed, she accidentally killed the twins and rushed to regroup with them.

While Maple was sick, Mai and the other Maple Tree members advanced to the 5th layer without her.

Mai and Yui helped Maple kill the King of Light in about four pairs of attacks and learned the [Light Arrow] skill.

Tower Arc
Yui, Chrome, Iz, Kasumi, Kanade, and Mai worked together to climb the 7th event's tower. Mai and Yui's attack power allowed them to overcome many obstacles easily.

On the 5th floor, they changed their strategy because monsters with physical immunity became common. The twins were separated from the rest of the group and devised a combination attack they used to pass through the mist safely. After they regrouped with the others, they moved on and easily defeated the boss.

When Maple and Sally had just finished the 7th floor, Mai and the others had already cleared the 9th floor and had been defeated by the 10th floor's boss.

Companions Arc
Mai and Yui went searching for pets with Maple. They spotted a bear cub inside a forest but couldn't keep up with it. After Maple left, the twins studied the bear cub and others monsters in the forest. During a break, Iz gave them some items to aid them. Their persistence allowed them to get bear cub Monster companion. Later at the guild, the twins recolored their companions with items Iz gave them and named them Tsukimi and Yukimi.

Eighth Event Arc
Mai and Yui participated in the 8th event. It was implied that Yui and Yukimi defeated many monsters and players together. After Yui noticed Maple's location on the map, the twins met up near her location before they headed out together. They ended up leaving Maple alone when they saw she was doing fine.

During the main event, she and Mai joined up with Sally and Maple. They were a great help in taking down the mud doll boss. When they were seperated Yui ran into Kanade and Drag. They ended up teaming up until Sally, Frederica, Payne and Dred found them. Mai and Yui then joined Dred and Marx to take down a dungeon, avoided most of the monsters thanks to Shadow and took down a large Electric Catfish boss. On the third day, the three guilds took down Cthulhu together.