They can make it easier to remember. For example, if you know the origin of '人' (person) as a simple drawing of a person standing, it sticks in your mind better.
The kanji '日' (sun) has an origin that's quite straightforward. It looks like a circle with a dot in the middle, which is thought to be a representation of the sun. In ancient times, when people were creating kanji, they used such simple and intuitive ways to depict common elements in their environment. The shape is so recognizable that it's easy to understand its origin.
Well, kanji is vital. In light novels, it can enhance the aesthetic appeal. Readers who are familiar with kanji can quickly grasp the intended meaning. Moreover, kanji is often used for names of characters, places, and important items in the story. Without kanji, it would be difficult to create unique and memorable names, and the overall feel of the light novel would be less distinct.
There are several. Kanji for directions like 東 (higashi, 'east'), 西 (nishi, 'west'), 南 (minami,'south') and 北 (kita, 'north') can be used when describing a place or a journey. Kanji for numbers are also common. For example, 一 (ichi, 'one'), 二 (ni, 'two') etc. Also, kanji for colors such as 赤 (aka,'red') and 青 (ao, 'blue') are often found in light novels to describe characters' appearances or the scenery.
Not always. Many manga have furigana (small hiragana or katakana above kanji) to help with pronunciation and understanding. Also, some simple manga are written mostly in hiragana and katakana.
One interesting Yu x Kanji fanfiction could be about their adventures in a magical world. They might encounter strange creatures and have to use their unique skills, with Yu's bravery and Kanji's intelligence to overcome challenges.