Yuta Okkotsu is the main character of Jujitsu Kaisen 0 and a supporting character of Jujitsu Kaisen.
His story comes from his childhood, befriending someone named Rika, who then promised to marry each other, and commemorated it with a ring. Unfortunately, that was the last Yuta would see of her, as she was involved in a car crash that ended her life.
Overcome with an unfathomable amount of grief, Yuta somehow manifested the pure love and grief she had for her after her death into a curse — one that manifested as a monster built from curse energy called a shikigami.
The strength of Yuta's shikigami is built only on love, with that love being so strong that it makes the shikigami's strength rival that of some of the strongest in the world.
Additionally, having joined jujitsu society, he was known for being a prodigy, completing most of its syllabus in one year, and has the largest cursed energy output in Modern Jujitsu Kaisen history, bested only by Ryomen Sukuna himself.