Icarus Toma is Eagle Marin's younger brother, that she lost years ago, before becoming a Silver Saint. In the Next Dimension manga, he's imprisoned for all this time in the Heaven, and that's why he accepts Callisto's order to kill Pegasus Seiya, trying to get his own freedom.

In the movie The Heaven Chapter - Prelude, there's not much explanation about him and his motives. The Icarus Glory, comparing the manga and the movie versions, are also very different.