I think that when Pro. Clay said that Mira's brother is gone, I think he meant his mind is gone, but his body is still alive, in the form of Spectra.
i don't think it is spectra, the hair color changes, but then again alices changed drastically, I'm thinking that her brother is prince (i-forget-his-name) the guy who always plays with his hair and collected the 5/6 savior bakugan, i could be going of on the wrong track here, im putting my two cents in. i would like the conflict that the show would get if mira's brother is spectra tho, ill be expecting that, either that or prof. clay used mira's brother in an experiment of some kind and he was killed or something, mira's brother was in the lab that day that she heard hydranoid talk when he was fighting so maybe hydranoid hurt him or something, who know at this point.