Romance. Cosmo became good friends with Sonic and friends, but most particularly with her boyfriend Tails. The two of them ended up alone on a mission together at one point when Tails got hurt, Cosmo stiched his bruise and Tails blushes in return. Later on, when the Blue Typhoon meets the Chaotix for the second time.
At the end If you was nice to Amy, she says that her boyfriend called Dexter is actually invited and created by Amy to make Sonic jealous and that she’s so happy that Sonic has feelings for her. You can say that all the Sonic and Amy moments are made for the fans and that Sonic is really interested in Amy.