Sytry first met Solomon in a library after he had fallen from heaven. As Solomon gave him affection and a home in hell, Sytry apparently developed feelings for him. Sytry, along with many other demons, were deeply saddened when Solomon was killed.
At the start of the series, Sytry was only interested in William because he was the elector. However, as the story progresses Sytry becomes closer to William, as shown when he left the demon party to save him from Baalberith's forces.
Sytry becomes very protective of William after finding out that Dantalion had killed Solomon.
Dantalion and Sytry have a very poor relationship due to Sytry's hatred of Nephilims and Dantalion's disgust at Sytry being Baalberith's "puppet". Their relationship got worse when Sytry and Camio found out that Dantalion had killed Solomon, causing Sytry to declare that he will keep William away from Dantalion and the two of them to engage in a fight.
Neither seem that interested in each other as they are not shown interacting very often.
Kevin Cecil Edit
The two weren't seen interacting before Dantalion vanished. Afterwards, when Sytry tried to convince William to choose him, Kevin threatened him, causing him to call Kevin a sadistic angel.