Cassie

Meeting Cassie
Cassie can be met shortly after escaping from Ruined Lab. She would be at the warehouse in eastern part of the city. She is distrustful at first, so player needs either convince her (Speech skill-check) or defeat her in battle. Either talking her down or defeating her convinces her what you are not her enemy, and she warms to you. After you talk to her again, she tells how to reach New Ark.
 * Cassie would not trust someone who tries to convince her of being her friend while being armed to the teeth. Make sure to de-equip your weapons.
 * Failing Speech check several times leads to her becoming completely distrustful, disabling non-violent option.

Befriending Cassie
This task is rather simple. You need to give her something to eat -- namely, fish. The bigger fish is, the more thankful Cassie would be, but otherwise effect would be the same. She wouldn't accept Weird fish. Free Fishing rod can be found in the store to the north from Ruined Lab's entrance, and good place for fishing is available near warehouse. Fishing is Senses skill-check, so keep trying until you got it. Using Worms makes things easier and allows to find bigger fishes. Worms can be bought at Slum's market (see below how to reach it), or they just respawn in random amount on playground each time you rest.

After she befriends you, she tells you how to find her hideout, unlocking it.

Don't screw with Don
Since you likely don't have any money on you yet, you can't just buy a passage. However, guy standing nearby talks something about other way — through the sewers. Eww. After following through the sewers, Lust Doll reaches Slums.

The only exit from here is guarded by toll guy who demands $5 for every passage. To progress further, Lust Doll must somehow get past him without paying — either by having sufficient Strength to push him, or sufficient Speech to trick him.

After resting and revisiting Slums, Don's guys would find you and bring back to Don, who would demand not just the money you didn't pay, but also "fee", resulting in $200 "fine" this time. Paying up would just reset him, so to progress up you need to refuse and offer to work it up instead.

Don doesn't need assassins, so it would be either sex work or grunt work. Sex work opens additional scenes (this is repeatable event as long as you don't progress further on this quest), but story would only progress further after you accept grunt work. Choosing to resist at the start, but backing up after Don admits what you have guts would result in you automatically picking grunt work.

"Grunt work" is simple – find one particular cat-girl, and bring her back. Despite all attempts to warn Cassie, this amounts to nothing: Don decided to not trust you, and just sent his men to follow you and learn where Cassie hides; after you rest, Cassie would be abducted.

You can accept or refuse your payment ($500), but it wouldn't affect subsequent plot in any way.

Finding friends
You can't deal with Don you your own; firstly, you need some friends.

Firstly, go to Condo and rent room here. After resting here, Lust Doll would meet their new neighbour, girl called Fawn. Keep talking with Fawn (you need to rest after each meeting) until she invites you in her room, and leaves you alone (this event is repeatable as long as you don't progress further through this quest). Search the room until you find the membership card of some place called... "The Torture Rack"?..

Go to "The Torture Rack" (it's in the Slums), and try to learn more by asking receptionist. Unfortunately, they would tell you nothing since they protect their members' privacy. You may sell the membership card, or try to bust Fawn exiting the club. Either way, she would confront you later and just tell what yes, she works at BDSM club, and would offer you to become her personal sub (starting her own quest-line).

After telling her about Cassie and Don, she would reveal what Don is actually one of her club's patrons.

After resting, visit Fawn again and discuss with her possible ways to deal with Don. After talking about each possible option, you would come up with plan of actions.

Without police involvement, saving Cassie would be impossible. Visit local Police station and talk with the girl called Arlene. Unfortunately, police has sort of... "agreement" with Don: he keeps Slums in check, police doesn't bother him; and so far he maintained his part of bargain. So far...

Visit Slums again, and find local homeless. They would demand lots of money, but don't just agree on the first sum they ask for: they would just steal it. Keep lowering the amount you are willing to pay (losing supporters each time), until everyone who are just here for free money would go away, leaving only people willing to risk their lives (they would demand $500 for their efforts). They would spark a rebellion, spoiling Don's relationship with police and allowing Arlene to intervene.

From now on, you are technically ready to bust in, but you can make it even easier by recruiting either Lin-Lin or Sammy. Recruiting both seems to do nothing. Lin-Lin would agree to help for $300 without asking anything, while Sammy needs to be befriended first.

Rescue operation
There are several ways to infiltrate Don's hideout, but all of them involve hard skill-checks. Positive Status effects seems to do nothing, with skill-checks checking for only basic values.

You may steal the keys and try to sneak through, or you may deal with guards violently. Failing to do either results in a Bad End.

After you are inside, you can try to bust through, or sneak past. Failing to do either result in a Bad End.

After you reach Cassie, you confront Don. If you previously recruited Arlene, she would arrest Don, otherwise, there would be no non-lethal way to deal with problem, creating power vacuum and resulting in Bad End.