Ap bruisers DO exist, they're just not defined by their items the way AD bruiser are
With the recent outrage over Galio building AP and dealing significant amounts of damage people have been going on how Bruiser Galio will never work, stating how AP fighters just lack the tools to itemize for their play style. Ad bruisers can just get Trinity force, Black cleaver and titanic hydra and deal significant DPS because they can stack "free" damage without having to invest heavily into one offensive stat.
Yet this does not mean that Ap bruisers are doomed to fail as I strongly believe that a champion's niche is defined by their play style. AP Bruisers are basically just melee battlemages as they're mostly short/medium range damage threats and historically have just picked up one or two defensive AP items.
are just the same as
as they're all of the same vein, being sturdy AP damage threats.
The melee champions I listed go almost entirely FULL AP, with the exception of Mordekaiser who adds some hybrid items to his item pool and yet they're undeniably Bruisers as they thrive in scrappy fights and have to get into melee range to deal some of their damage. Do they have an AP black cleaver? Do they have a trinity force to add small chunks of dps to their combat pattern?
These champions build heavy AP and still function as fighters because a lot of their durability scales directly off AP. Be it shields, heals or damage reduction, it all scales off AP and when paired with some HP and resistances these champions become really durable despite usually building mostly offensive items.
Galio himself works as a bruiser, with builds like
giving him enough damage, survivability and CDR to fulfill his niche as a durable AP damage threat.
So, next time people say Galio building AP doesn't make him a bruiser think about all the other champions who share a niche with him.
~ A mildly annoyed Galio main

are the only real AP bruisers in my books. The problem is that they are not really that well defined like AD champs are.