Miranda states that Niket is the only friend she had while living with her father and the only person she kept in contact with when she fled. He's also one of the few who understands what her father is really like. However, Niket grew frustrated with Miranda after finding out that she joined Cerberus and stole her sister without so much as informing him.
He betrays Miranda, believing she kidnapped Oriana for revenge against her father. He argues that Oriana would be better off wealthy rather than growing up in poverty like himself. The night Miranda revealed that she stole her sister to him, Niket called her father. Miranda's father bought him out, on the condition he set the terms of the transfer. He wanted to carefully arrange for Oriana's return to her father, not willing to traumatize her adoptive family in the process.
Up to the last minute, Miranda refuses to believe that Niket turned on her. Niket had been known to shorten her name to "Miri", and she was sure her father would've tried to buy him off back when she ran away. Because he kept all other information about Oriana from her father, Miranda, without Shepard's intervention, will kill him to tie up loose ends. Otherwise Captain Enyala will kill him herself.