Pastille, despite being clearly considered by Syrup to be just a lowly assistant, is really the one who keeps Atelier Sweets running by overseeing finances and being friendly to customers who would've been put off by Syrup's asocial demeanor. Syrup has some serious problems interacting with people, and for a long time Pastille seems to be the only person who tolerates her.
Despite Syrup's behaviour, Pastille still tries his best to not show her in a bad light because he knows she's a good person