Cosmo and Sprout have been best friends since childhood. Both of them were in high school and in the 11th grade. Despite being best friends,

Cosmo was more cold and often throws insults at Sprout. Sprout knew he could handle it but it just hurts… As much as he wanted to get through with it, he didn’t know how much longer he could handle it.

So, he started hanging out with his other friend, Finn, more as a temptation to balance things out with Cosmo.