Saul grew up with people afraid of him and his magic, since he is the son of a witch and a demon. He has magic, but he rarely uses it because it's unstable and only comes in random bursts. He has younger cousins, Guy and Olivander, that he spent a lot of time with, thinking of them as his younger brothers but also almost his sons.
He was 17 when he met the 12 year old witch, Letty. They touched hands at a party, revealing they had a soulmate connection. Because it had nothing to do with a kiss, Saul told Letty the connection between them was platonic.
When Letty 'died,' he saw his cousins wasting away trying to find her. The dimension Letty was trapped in had blocked her soulmate connection to Saul, removing his ability to feel her magic. He didn't want to believe she was dead, in fact, he believed the opposite, but with Guy and Olivander practically killing themselves with worry, he told them to accept that Letty was dead before leaving them behind. All he wanted was for his cousins to have some form of happiness without Letty, even though it killed him to leave them when they were hurting.
He was very drunk one night when he felt Letty's magic again, at the same moment that the twins helped her temporarily escape from the Asylum. It gave him hope, but he still stayed away in the hopes it would encourage his cousins to heal and move on.
12 years after Letty was taken, the twins found where he was living in a remote village in Europe, and begged him to give them some of his blood for a tracking spell that may or may not work to find Letty. He hates that they're still looking for her, but that single moment of feeling her makes him relent and give the blood, but only if they promise they'll stop if this doesn't work. They swear, he gives the blood, and they leave to find Letty.
Tales of the Favored: A Christmas Tale