You could say "tu prim/a está loco/a", depending on the gender of the counsin.
mi amigo loco
Lo siento, mi amigo, no soy loco/a (male/female 'I')
Bésame el trasero that's how u say you are crazy about me
loco
You can say "Estoy loco(a)".
la vida loca
¿Eres loco?
Me vuelves loco/a.
No estoy loco
Estamos locos
You say, "tortuga loca."
Crazy friend (fem): amie folle Crazy friend (masc): ami fou Crazy mate (fem): copine folle Crazy mate (masc): copain fou (:
To say hello friend in Spanish you can say "hola amigo".