this message may be
offensive
When it's revealed Ruin was actually a bad guy(not going to get into Ruin’s psychology too right now) and collapsed over 5000 dimensions, including Solar's, causing him to vanish out of existence as Nexus held him. When Eclipse brought Solar back from the dead, he had a new body and was told that Nexus went insane and tried to kill Earth and the original Moon was back. Solar had process that his best friend was apparently evil now and was in denial of it until Nexus came to talk to him personally. Nexus offered Solar to join him, and when he denied(still not knowing Nexus's side of things btw)Nexus threatened the family, making a specific point on Jack and Dazzle. Solar decided to side with the rest of the family and was first to say Nexus had to die. Then the plot focused in on Moon for a bit, but Solar did say some things that implied he may be struggling with depressive feelings but always quickly changed the subject so it wouldn't get lingered on. Solar used Moon's simulation system thing a couple times; first with alternative timeline where he never died and Nexus never 'lost it', and again more recently to stimulate a timeline where his dimension's Sun never died. Then Nexus kidnapped Moon, and Solar and Sun went to save him. They ran into Nexus who tried to kill Solar, Moon stopped him and Nexus went to kill Moon instead, only for Sun to kill Nexus instead. Solar didn't seem to linger on Nexus's death and instead focused on Sun's mental state. Later Solar tried to convince Moon to comfort Sun but he just buried himself in his work, Solar tried again in a later episode without success, and that brings us to the present.
God damn, that took over an hour to type.
tldr; Solar is traumatised to hell and back and needs a breakdown arc