At first, she was a bit distrustful of him, due to his origin of Xenecromorph blood, as they call the Black Ooze. However, once she got a good look at the molecular structure of his nanotech body, seeing a perfect fusion between flesh and machine... she was enthralled completely. And once she first met him... it was love at first sight for her.
Because of her love for him, she's completely devoted to DRU, and would do anything for him. His dense nature towards her goes as hinted he really does know about her feelings for him, but feels like it wouldn't work between the two. Though it shows that the two have been dating and making out in secret despite the others knowing this.
The rivalry between Rei and Salu is probably the most apparent in the story. This rivalry is not really taken seriously, as it is one-sided, with only Salu considering Rei as her 'unforgivable rival in love' in regards to DRU and Rei being rather oblivious.
Even though Salu still considered Rei as her rival in love, both of them cared for each other deeply. They showed a strong bond together.
Rei is also one of the first people who greeted Salu when she first entered the crew, although she was completely shut down by her, who still thought Rei as a love rival. Despite their bond, Salu still attempts to sabotage Rei (although usually in a comical way), especially if the android gets too close to the nanomorph.