Quoted By:
To me, a bro is what you you've pushed the limits with and had a good reliance on whether its a normal competitive battle, in-game run or a nuzlocke, something that you used which also served you well and for that reason on being reliable as fuck, it earns a right as one of your bros. Pic related as an example, versatile and has come in clutch a lot.
Ace is what you consider your most effective mon, can stem from a bromon usually but its that one overall you find yourself coming back to using and having it as your top fighter. Darmanitans pretty much my ace.
Shouldermons pretty straightforward, something small you can carry about which you also like. For me, that's Pumpkaboo.