Aaron may be useful when you are low level. As soon as you get over 30% crit, Aaron becomes useless. Shade's uptime overall is way better and with your high base crit you can reach 50~60~70% almost every 5,5s depending on it.
These are the baseline passive cards all classes can use:

Megiddon for Stamina regeneration and Shade for Crit.
If your class uses atk speed as base (Erwin, Stella, Haru post-advancement) or you want to make fast basic attacks to recover SF:

Atk speed given from Dark Gauntillus.
If your class doesn't use atk speed as base and you have SF problems or want to focus on unleashing skills with less basic attacks as possible:

SF regeneration from Grand Blaze Taifon.
If you don't have SF problems and you are soloing and you rely a lot on dodges:

Even more Stamina regeneration from Skitter.
The open slots are for actives.
If you are clearing trash content and want to do fast runs:

Baalride and Worker Ex Machina.
And the list goes on. You will change actives and passives all the time depending on what you play and how your style is.
For example, If you don't need SF neither you need atk speed you can even play with three actives (Grutin Army, Rita T1 and Big Plastica). That one is widely used when soloing Primal. And the moment you know all the patterns and are used to it, you can even drop Megiddon and add something else like Catherine (not out yet) that gives you +17% dmg on bosses. You can even drop Shade if you reach astounding % levels of crit at some point with Primal equipment.
All in all, sometimes you will use only 1 active, others you will ending up with 4 actives unleashing hell. All depends on where you are. And you will end up saying your farewell to Aaron pretty soon. Or not, if you don't have anything better to add to your roster.