They've known each other ever since Simon was appointed as Elias' watchdog 10 years prior to the start of the series. Chise describes them as being on bad terms with one another but as the series goes on we gradually see that while their relationships is a bit rocky, they understand each other quite well. They frequently take jabs at one another but they're also commonly seen joking with each other as well. Simon is also one of the few people that also understand Elias' mood and can easily tell when there is something wrong with him.
It's noted that all of the previous Church overseers have been driven away by Elias prior to him.
Even from the beginning, Simon showed kindness towards Chise and offered her a place at his church should she ever wanna get away from Elias. They seem to enjoy each other's company and Simon has shown to care for her as he has repeatedly made visits whenever Chise manages to get herself into trouble, being obviously worried about her well-being.
While they haven't made many interactions with one another, they clearly don't get along well as Silver has refused to let him into the house during numerous occasions. She has thus far only allowed it once but that's because Elias was the one who allowed him inside, even then she wasn't pleased when Simon was patting Chise's head as she was having her arms around her. It's so far unknown if there's a special reason for why she doesn't like him or if it's just because he's a priest.