Well, one possibility is that the star UUA is like a guardian of the apple. The star's rays might protect the apple from harm, like pests or bad weather. Another aspect could be that the apple is a sacrifice or an offering to the star UUA in the story's mythology. It could also be that the apple and the star UUA are linked through a prophecy. For instance, when the apple ripens fully, it could signal an important event related to the star, such as the star coming closer to the Earth or some sort of cosmic alignment.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
Well, here's a story. UUA was a star that had been observing the Earth for centuries. One day, it noticed an apple tree with the most delicious - looking apples. Among those apples, there was one that stood out. It was big and shiny. The star UUA was so intrigued that it sent a little bit of its starlight down to touch the apple. The apple then became filled with a special energy. Whenever someone would look at the apple, they could see a faint reflection of the star UUA in it.