|Via Reboot Wiki|
Mouse immediately finds Ray attractive and pursues him, because, well, that's who Mouse is; she behaved similarly with Bob until she developed a friendship with Dot. Ray is impressed with Mouse's hacking skills, and as they fly around Mainframe, trying to help the denizens of this dying system (at this point, it was taken over by Megabyte and Hex), they got even closer, to the point where Ray even offers to take her away.
It's the chemistry between the two characters that get me. Mouse is as free-spirited as they come, and Ray is pretty much an Australian* beach bum, going wherever the Web takes him. There's no indication that this relationship is going to last much longer than their random passions, but I don't think either cares: it's all about the moment for both of them. It's an interesting juxtaposition to Dot and Bob and Matrix and AndrAIa, two long-standing relationships that are based on completely different things.
Basically, what I'm saying is ... Mouse and Ray forever, unless they decide to amicably part ways and then meet up for casual sex whenever they're in the mood.
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* Ray Tracer is voiced by Donal Gibson, Mel Gibson's younger - and arguably less crazy - brother.