YumiHisu

Yumikuri is the femslash ship between Ymir and Krista Lenz from the Attack on Titan fandom.

Canon
Ymir and Krista both enlisted in the 104th cadet training corps. The two of them developed a close friendship and Ymir was romantically attracted to Krista, sometimes referring to her as 'sweetie'.

Despite her normally cold and apathetic demeanour, Ymir cared deeply about Krista and even helped her graduate in the Top 10 so she could live safely in the Interior as part of the Military Police.

Ymir saw through Krista's 'good girl' act and encouraged Krista to embrace her true self and be proud of it. This inspired Krista to reveal her true name, Historia Reiss.

When they were surrounded by titans Historia told Ymir that, as long as they are together, she is not afraid.

After Eren utilised his titan-controlling ability, Ymir figured that Historia would be safe behind the walls and this gave her a sense of peace, allowing her to willingly lay down her life to pay back the debt she owed Reiner and Bertolt.

Fanon
Considering the pairing's canonicity, it is probable that parts of the relationship between Ymir and Historia were left out of the story. The two of them were likely more affectionate with each other than we got to see.