Reiner did die in the manga. His death was a result of a series of events and conflicts that had been building up throughout the storyline. It had a major impact on the plot and the other characters.
One key element is definitely Ymir's emergence from the chaos of Ginnungagap. Then there's the significance of his body being used to form the world. His bones for mountains, his flesh for land, all these are important elements that show how the Norse believed the world was created from this one giant being. It also implies a certain hierarchy in the cosmos as the later - arriving gods were the ones who shaped the world from Ymir.
In a fanfic, Christa and Ymir might find a hidden place within the walls. This place has secrets about the origin of the Titans. As they explore it, they learn more about each other. Christa could discover Ymir's past and understand why she is the way she is. Their story could be filled with mystery and adventure, and in the end, they become even closer friends.
If Lord Reiner is from a particular book or game, more context is needed. For example, if it's a character from a fantasy novel, we would need to know the plot of the novel, the world it is set in, and his relationships with other characters to tell his full story.
Ymir is often considered the first being in Norse mythology. According to the origin story, Ymir emerged from the primordial chaos, specifically from the place where ice and fire met in Ginnungagap. Ymir was a giant, and from his body, other elements of the Norse cosmos were created. For example, his sweat became the sea, his bones became mountains, and his hair became trees. This origin story is fundamental in understanding the Norse view of creation and the relationships between different elements in their cosmology.
There could be stories about how he dealt with new musicians or young talent. His high standards might have been overwhelming for those just starting out. He expected them to quickly adapt to his way of working, and if they didn't, it could lead to some rather unpleasant experiences for the budding musicians.