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Traits give a one time bonus and can also apply a modifier every time they're pimped out. If a girl has shit traits, she can have all her stats take a massive nose dive every sex attempt. If she has great traits, she can actually gain stats she'd normally lose like Men. Traits are super important if you want to keep a girl healthy and you should try to protect the really awesome ones. You lose 200 Looks and 800 Men during normal sex, so traits that raise those should be prioritized.
Edit: It seems the wiki has started updating some of these with the English translations, so the totals might be less needed soon. A few of these are assumed, because they're a pain in the ass to find. It's at the point where they're pretty safe assumptions though but I'll keep on the look out.
Quoting this as it's useful information, but also to add my own summary of the process to it
If you want to break the entire hustling aspect of the game over your knee, it's pretty easy. All you really need is a few girls with the following traits:
A combination of 2 from the following: Devious, Pure, Sexy, and Pervy
If Pervy is one of the two, then the third trait should be from one of the following: Healing, Impish, or Obedient
Otherwise, Chipper, Cool, Devoted, Just, Mystic, or Stubborn will do
Pervy is the only one that negatively impacts Looks development so you want to offset that with one of the three I mentioned. Unfortunately it means that Looks development will be neutral, which means you'll have to rely on items or Looks bonuses from clients to pump that stat up afterward. If you avoid Pervy and use two of the remaining three instead, then fucking your own prostitutes will develop their Looks and Tech rapidly. Do this with a handful of girls (3-6) and you'll pretty much never have to worry about money again
You can do this pretty early in the game. After clearing the gallery is maybe the easiest early point because the girls you can dohna dohna from there tend to have better stats. Ideally you find a girl with B or higher looks and no default traits, but if they have one unwanted trait that's manageable. Then, save, go hustle, and reset if none of the clients have one of the traits you want to put on the girl. This might take a while, but eventually you'll find and snag someone, so just save and repeat the following day with another trait. It's best to capture all the girls you'll want to train first since some traits appear way more frequently than others (this was the case with Pure for me) so you can develop multiple girls at once if you stumble upon a trait you've already put on someone. Also make sure you have at least one girl with a D in looks so you can bring her out to avoid clients you don't want to inherit traits from
If you're training a girl who already has one unwanted trait, that's fine, but can slow down the process. Once they have three traits inheriting another will overwrite one of the three at random, so you can use that to get rid of the unwanted trait. It's just going to require a lot more resetting to get the trait you want and to have it replace the right slot. It's also best to do this whole process on Appreciation/Desperation days, since the larger pool of clients means a better chance at least one person will have something you want, but it's not necessary as all of the desired traits can be found on any day
The whole process will probably take a handful of hours but once it's done you'll be able to rake in money effortlessly. The only thing you'll have to do periodically is hunt the hospital for birth control pills, but you'll also be earning enough that you can buy a lot of them from the shop if you check it regularly. With this method you also won't really need to bother with the base upgrades that improve the effectiveness of items. Only the upgrades that give you more rooms will be useful so you can recruit the unique prostitutes and do their events. Also the bonuses to TP help you build their stats up faster by fucking them more often per day, but it's not a big deal
The one final note I have is that it's worth having a variety of Mental levels for the girls since the VIP clients have a range of preferences. S+ in Looks and Tech is too valuable to give up, but since these combinations of traits make it so that their Mental stat doesn't change from prostitution then you can ignore it or build it up with items as you please
Anyway, hope that helps. Happy hustling