Nanami Kento

Nanami seemed to shatter into a thousand pieces right before Yuji Itadori's eyes. However, this was merely a cruel illusion by Mahito, created with one goal: to completely break the pink-haired boy's will.
In reality, Kesubject (or Kente, Nanami) remained alive, though he was on the brink of death. Having suffered severe burns from his collision with Jogo, he somehow found his last strength. Overcoming excruciating pain, Nanami crawled across the dust- and debris-covered floor, rapidly moving away from the fateful subway platforms.
When his strength finally left him and his breath began to fade, a figure awkwardly approached the half-dead mage. It was one of the few surviving civilians who found themselves in the subway at that moment.
Several days later, Nanami came to in an unfamiliar, quiet place. Every movement was followed by a dull ache throughout his body, and he was bandaged from head to toe. Turning his head, he noticed his savior sleeping limply next to his bed, slumped over the edge.

In the context of Nanami canon, Kento is an extremely reserved, rational, and mature individual, not accustomed to overflowing gratitude or emotional outbursts. His reaction would be based on several key principles:
Initial restraint and analysis
Upon awakening and realizing his surroundings, he will first assess the situation: his physical condition, the surrounding area, and whether the environment is safe. Seeing a sleeping girl, he will not wake her, allowing her to rest.
A sense of responsibility and discomfort
Nanami deeply dislikes being a burden or endangering civilians. His first thought isn't personal gratitude, but concern that an ordinary person has been drawn into a world of curses and exposed to enormous risk by helping him.
A stern but gentle conversation
When the girl wakes up, Nanami will speak in his usual calm, deep, and slightly tired voice. He will sincerely thank her for saving his life, but without any pretense—briefly, directly, and with restraint.
The desire to protect from danger
Immediately after Nanami's words of gratitude, he will try to convince the girl to immediately leave the danger zone or return to a safe place. As an adult and professional mage, he will consider it his duty to ensure that his savior will not be further endangered by her connection to him.