Solomon presents himself as confident and knowledgeable. Despite presumably only arriving in the Devildom recently, he exudes confidence in his actions and seems to understand most of what goes on in the RAD (Royal Academy of Diavolo) it's school. His confidence may also be a result of his magic, which is said by Lucifer to be quite powerful. He is terminally ill and he's worried when he tell you this, you'll be mad at him. He's hide it from you.