Solar Eclipse is a calm and calculating alicorn. He is the negative counterpart of Axis Solaris.
In Positive Equestria, Solar Eclipse looks almost exactly like Solaris, despite being older. When he is in Negative Equestria, however, he is dark orange with a black mane and tail. He has silver eyes with slits and appears much taller than Solaris.
Solar Eclipse, in contrast to Solaris, is calm, takes in situations, and thinks before he speaks and attacks. He moves slowly, but makes up for it with strong magical power. He keeps his sister in check by comparing her to his Positive counterpart.
The son of Lunis and nephew of Celeste, Solar Eclipse was born five years before Lunis was killed. Celeste took over her sister's position and Solar was appointed second in command. During this, Solar studied ancient scrolls. He learned of a prophecy that his world would crumble. There seemed to be no hope until he found another set of scrolls that stated there was a parallel world to his. When Celeste passed he took the throne with his sister. Over the next few years they sent the citizens of their world to harvest the supplies they needed to transport between worlds. In the end they created The Cloudy Mirror. The world was showing signs of collapse, so Solar transported himself to Equestria with the mirror.
Early in the story, Solar Eclipse travels around Equestria, taking on Solaris's form and name. He uses this to try and frame Solaris, therefore flushing him out while trying to find the real culprit. Once Solaris is out in the open, Solar captures him and sends him to Negative Equestria with the mirror.
Solar then uses his form of Solaris to gain access to the special royal scrolls, trying to find a way to prevent the collapse of Negative Equestria. He doesn't find a way to save the world, but he does find a way to use Equestria's stable energy, take it, and use it for Negative Equestria, even though it will cause the collapse of Equestria.