People really use the term rival so easily these days...
Coby rivals luffy when it's actually Blackbeard
Helmeppo rivals Zoro when it's actually just straight up delusion
And, Coby is and will always be fanboy and not a rival.
Dude owes everything to Luffy.
Luffy saved his ass and inspired him to go after his dream.
Luffy's Grandpa trained him
Luffy by having constant achievements is pushing Coby more
And, most importantly, Coby uses Luffy as conversation topic to chill out with girls
Drools and cries seeing Luffy in newspaper
I won't be surprised if Coby after getting a child name him "luffy"