They were both presented as being Fonzie’s nephews, right? Besides that, I remember Spike as being a prepubescent boy, with much more of a Fonzie-hero-worship thing going on; he wore the same leather jacket, t-shirt and dungarees.
Which Chachi didn’t, and Chachi was older, too. But the show ran for a long time, so maybe he was re-introduced with some explanation for the name change, much like Mike Doonesbury’s baby brother Benjy was later brought back as college student Sal Doonesbury?
Indeed, Spike was on the show during the first couple of seasons. Chachi came along a couple of years later. Spike was Fonzi’s nephew, Chachi was his cousin.
He’s not listed on the IMDB cast list for Happy Days.
Spike’s first appearance was in “Not With My Sister, You Don’t” (ep. 2.8) on 19 Nov. 1974. Scott Baio joined the cast in '77, same year that Danny Butch made his last appearance as Spike.