Who is Transistor or Mr.Nobody?

[Linstimpy]

I think he is Red's boyfriend but i don't know maybe just a normal person?

[>:^O]

They call him "Blue", and I think you're right.

[aph1]

it's the sword that's talking

[Snarplord XVI]

I have a feeling he's the unknown trace from Breach(). But yeah he's Red's husband/boyfriend/lover/whatever.

[Melethiel Minastauriel]

He's Red's lover. I wouldn't call him "boyfriend", tho because it seems to me that they were still getting closer to eachother than being already in a relationship. That's because Sybil was jelous of him and attempted to kill Red while knowing his lover would protect her.
In other words, Sybil used the assassination attempt to get rid of that guy and have Red all for herself. I don't know if you have already finished the game but, [SPOILERS AHEAD] in the ending cutscene, you can see various drawings in which the two are together.

Also, he is indeed the man described in the Breach() skill. Infact, if you unlock all three slots (active, passive and add-on), the narrator says that the man described it's him.

[ju1i3]

I think they were already pretty close... How they miss each other during the story and seem to have some 'history' and intimacy already. [SPOILERS FROM NOW ON] And, as you said, in the ending cutscene where there are many images of the two, togheter, kinda of doing couple stuff.

Funny, i didn't noticed he revealed himself when Breach was fully unlocked, just watched a video here to remember the phrase, and indirectly he really reveals himself. "You've reached the hopefully-not-permanent address of Mr. Nobody" :p