Where the One at times in these revelations might appear to be God the Father (Revelations 4:2), the mention of the twelve in verse 10 clearly marks Christ as the "One" indicated in this vision for this occasion.
The brightness of the twelve (likely including the "One" in the description) is that their brightness exceeded that of the stars. Shining, whiteness, and brightness are typical descriptions of beings which appear in their glory. The comparison of that brightness to *stars* however, is fairly unusual, as the stars are fairly low on the brightness scale of the heavenly objects observable from the earth. This is either a ritualistic phrase, or the very idea of the stars is given more for the celestial correlation than the degree of brightness.
The inclusion of the 12 in this vision - long before their calling, is indicative of the premortal foreordination of the 12, and the connections with the apostles and the tribes of Israel.