I know people like to play him as an assassin, but everything in his kit is built for a bruiser.
He would play more like a Diana or an irelia as opposed to a bursty assassin, but that's what's most successful on him anyway.
The only thing that even allows Ekko to be a decent skirmisher are his passive and his W passive. He was intended to be an assassin, but people still build him with tanky AP items or as a true tank because his W passive is busted. And I don't care what you think about Diana or what Riot says about her, the bitch is an ASSASSIN. Just because she relies on auto attacks more than most assassins, that doesn't make her a fighter/skirmisher. She still melts squishy targets just as well as any other assassin.
If you think about Ekko in the same way, you'll understand that he's still an assassin. His combo just takes longer to execute because he relies on autos to activate his W passive damage and because a tanky AP item won't give you as much burst as a true damage item like a Deathcap.