Sahara had always been there for Nuka, from when they were cubs. She was the daughter of one of Dotti's sisters. Her birth had been unusual, as an only cub. Sahara always supported Nuka, in everything he did, despite being emaciated from Zira refusing to let the other mothers feed their cubs. Thankfully, her mother was an insomniac and fed her cub at night while Zira slept.
She was there when Zira expanded on the thought process Kovu had had and had only shaken her head. Sahara knew this would only end badly. And she was right. She had been forced to take part in the assault on the Pridelands and, consequently, was present when Nuka died because of Zira's rashness.
The malnourished lioness hurried over to her friend and cradled his head in her paw since Zira had taken off after Simba instead of tending to her dying son. Sahara remained by his side as he lay dying, talking with him softly and remembering when they were younger.
"I'll always be there for you, Nuka. I'll never leave your side."
"Promise?"
"Promise. I will never leave you."
Sahara sighed. "And I never did. Nuka, I'm so sorry..."
"I did it for you, Saha… ra..."
I never wanted you to, but thank you. Sahara thought and chose to remain with her friend when the rest of the pride reluctantly followed Zira now that Nuka was gone. Since they couldn't be together in life, they would unite in death.
Eventually, Sahara fell asleep for the last time and was reunited with her friend in the sky. And they watched over the uniting of prides and Kovu and Kiara becoming next in line for the throne.