Well, okay, yes, Jimenez remembers everything, but he often manages to make that a *bad* thing.
what's the deal with he and Cassie?
A bit complicated, and not completely explained. The thing with Ares is, WML (the guy who took over the series after Perez) either thought his reformation was boring (quite possible) or completely fucking missed the entire point of Diana's defeat of him (frankly more likely), and just made him a bad guy again right out of the gate, with no explanation whatsoever (and none has ever been given since). From WML on out, that characterization has been basically preserved - "Diana's antagonist," and while guys like Rucka and Jimenez gave it some nuance... well, the Cassie thing started with Johns, and "nuance" is not exactly his middle name, most especially when it comes to Wondy.
So what happened with Ares and Cassie is, after Donna died, Cassie was mopey, and Ares took advantage of this to show up in her life, give her a lasso and ask her to be his champion for no apparent reason. Cassie, as the first act of her astonishing bout of massive OOCness ongoing to this day, simply accepted his gifts and patronage with all the curiosity of a sleeping turtle. Eventually she asked him if maybe he was her father and that was why, but it turns out Zeus was her father, at which point she dropped it again. Then eventually, recently, she realized his power was bogus and rejected it. That's the full extent of that plotline. What's the deal with him and Cassie? Geoff Johns, that's the deal with him and Cassie.
If I were to fanwank it, though - that was the same time as Athena was starting her power games on Olympus, and before Athena had offered Ares the alliance. Trying to recruit Cassie as his own champion could have been a preemptive attempt to protect himself when the shit hit the fan - Athena having Diana in her corner was a major resource, and having Cassie in his corner would be the closest he could realistically get to matching it (plus it gives him an edge against Diana).