Don't open any Lockers with Keys as Haruka or you will have to wait until the story is over and Platinum Adventure is unlocked. (can still pick the keys up, just don't open lockers with them)

As neither the inventory or item box isn't shared, so any items are stuck until post game.

Also a quirk with this game in particular will not allow you to keep progress made there for a NG+ run so make anything before the point of no return in Kamurocho!
So it's permanently lost stuff if you wanted to use them in a ng+ Legend run.

You can make use of a trick to only use a single Mystery Stone: when you buy weapons the Mystery stone is only Required for it and won't be Consumed right away so you can use a single stone for all the weapons that need it if you buy them at the same time. don't keep more than a single stone in inventory or they will also be spent!

The Final tier of any Weapon (each cost 800,000 Yen!) is locked behind investing the Dragon type of that weapon and you need a Mystery Stone to buy them, the items you invest are:
Golden Dragon Bat
Divine Dragon Short Sword
Dragon Slayer
Mighty Dragon Spear
Raging Dragon Hammer
Twin Dragon Sticks
Dragon Horn
Dragon Nunchaku
Dragon Grudge Fist

You can also upgrade Shinadas Sturdy weapons, though you have to invest them first and each cost 100,000 Yen + a high tier weapon as material:
Sturdy Pipe: Colourful Umbrella. Atk 4->8
Material cost: Special police baton 58,000 -> Kamiyama Police baton 62,000 -> Colourful Umbrella 100,000 -> 220,000 + 100,000 thus only invest after you have 320,000 Yen

Sturdy Knife: Dragon Slayer Atk 5->8
Material cost:
Nameless Katana 4000 -> Sakura Storm 23,000 -> Photon Blade MG 20,000 -> Photon Blade PG 43,000 -> Dragon Slayer 200,000 -> 291,000 + 100,000 thus only invest after you have 391,000 Yen

Sturdy Pole: Mighty Dragon Spear Atk 5->6
Material cost:
Photon Blade WG 98,000 -> Mighty Dragon Spear 200,000 -> 298,000 + 100,000 thus only invest after you have 398,000 Yen

Sturdy Katana: Divine Dragon Short Sword Atk 5->9
Material cost:
Nameless short sword 4000 -> Spicy Knife 25,000 -> Chinese Broadsword 40,000 -> Lotus Clan Chinese Broadsword 60,000 -> Divine Dragon Short sword 200,000 -> 329,000 + 100,000 thus only invest after you have 429,000 Yen

Main Side Job

• Ash Wood - Reward for Garbage Collection,
  
• Rubber Ball - Reward for Garbage Collection,
  
• Chili Pepper - Reward for Garbage Collection,
  
• Durable Cloth - Reward for Garbage Collection
  
• Dragon Scale - Completion Reward for Taxi Driver

Sub Story

• High-Grade Platinum - Kiryu Substory "A Knock on the Door"

Store

• Aji Spicy Roe 240g (Regular) - Bought in Fukuya in Nagasugai
  
• Mystery Stone - Hostess Gacha Item,

IF8R (Unlocks near the end of Part 1 Chapter 4)

• Gravity Converter - Possible reward for using the IF8R, (Sotenbori Locker D2)
  
• Mystery Liquid - Possible reward for using the IF8R
  
• Small Motor - Possible reward for using the IF8R, (lottery)

Main Story/Side Job

• Block of Brass - Reward in Hunter and Killer
  
• Bloody Binding - Reward in Hunter and Killer, (Coliseum Item)
  
• Lighter Oil - Reward for a contract mission in Hunter and Killer, (lottery), (Don Quijote Sapporo)
  
• Magnetized Metal - Reward in Hunter and Killer
  
• Seal of the Dragon - Completion Reward for Hunter and Killer

Trainers

• Eye of the Dragon - Completion Reward for Tendo

Store

• Kamaitachi Utility Knife - Bought in Ebisu Pawn Tsukimino, (Kineicho), (Kamurocho), (The Cost of a Swing)
  
• Lighter Oil - Bought in Don Quijote Sapporo (Hunter and Killer), (lottery),
  
• Rubber Ball - Bought in Don Quijote Sapporo, (Garbage Collection)

IF8R

• Gravity Converter - Possible reward for using the IF8R, (Sotenbori Locker D2)
  
• Mystery Liquid - Possible reward for using the IF8R
  
• Small Motor - Possible reward for using the IF8R, (lottery)

Main Story/Side job
None

Sub story

• Fitness Gear - Akiyama Substory "Haggle Wars", (Kamurocho Locker B1)
  
• Durable Cloth - Akiyama Substory "Bad Boys" (Garbage Collection)

Store

• Battery - Bought at Don Quijote in Sotenbori, (Kamurocho)
  
• Chili Pepper - Bought in Don Quijote in Sotenbori, Kamurocho (Garbage Collection)

Locker Keys (Don't pick up as Haruka or you have to wait for postgame)

• Super Resilient Resin - Sotenbori Locker H3, (lottery)
  
• Gravity Converter - Sotenbori Locker D2, (IF8R)
  
• Super Resilient Resin - Sotenbori Locker H3, (lottery)
  
• Large Blow-Up Doll - In Sotenbori Locker C2, (lottery)
  
• Ash Wood - Sotenbori Locker F5 (Garbage Collection)

IF8R

• Gravity Converter - Possible reward for using the IF8R, (Sotenbori Locker D2)
  
• Mystery Liquid - Possible reward for using the IF8R
  
• Small Motor - Possible reward for using the IF8R, (lottery)

Main Story/ Side Job

• Sturdy Iron Pipe - Shinada automatically picks one up in his story. (Also from H1 Locker Kamurocho)
  
• Sturdy Pole - Reward for The Cost of a Swing, My Miracle Pitch
  
• Sturdy Knife - Reward for The Cost of a Swing, I Want To Be a Pro
  
• Sturdy Katana - Reward for The Cost of a Swing, Your Sign is Next
  
• Dragon Tail - Completion Reward for The Cost of a Swing
  
• Kamaitachi Utility Knife - Reward for The Cost of a Swing (Bought in Ebisu Pawn Tsukimino, Kineicho, Kamurocho)

Sub Story

• Cherry Tree Branch - Shinada Substory "Dirty Heart" and "Wanna Party?"
  
• Sacred Wood - Shinada Substory "Saving Yourself"
  
• High-Grade Platinum - Shinada Substory "A Precious Meal", (Kiryu Substory "A Knock on the Door") (Coliseum Item)

Store/Lottery

• Aoyagi Sweet Rice Cake 5 pcs - Bought from Dagaya in Kineicho
  
• Kamaitachi Utility Knife - Bought in Ebisu Pawn Kineicho, (Tsukimino), (Kamurocho), (The Cost of a Swing)
  
  
• Small Motor - possible reward for third place (blue) in Drawing Tickets lottery, (IF8R)
  
• Cherry Tree Branch -possible reward for third place (blue) in Drawing Tickets lottery, (Sotenbori Locker)
  
• Large Blow-Up Doll - possible reward for second place (white) in Drawing Tickets lottery, (Sotenbori Locker C2)
  
• Super Resilient Resin - possible reward for first place (red) in Drawing Tickets lottery (Sotenbori Locker)
  
• Marlin - possible reward for first place (red) in Drawing Tickets lottery (Fishing)
  Sea Fishing (Nagoya Wharf)
  
• Pearl - Reward for winning the Drawing Tickets lottery grand prize

Sea Fishing: Nagoya Wharf

• Marlin - Nagoya Sea Fishing, (lottery)
  
• Squid - Sea Fishing, (Sotenbori Locker)

IF8R

• Gravity Converter - Possible reward for using the IF8R, (Sotenbori Locker D2)
  
• Mystery Liquid - Possible reward for using the IF8R
  
• Small Motor - Possible reward for using the IF8R, (lottery)

Coliseum

• Dragon Talon - Coliseum Item
  
• Dragon Fang - Coliseum Item
  
• SSS Gunpowder - Coliseum Item, (Kamurocho Locker A2)
  
• Bloody Binding - Coliseum Item (Reward in Hunter and Killer),
  
• High-Grade Platinum - Coliseum Item (Kiryu Substory "A Knock on the Door" and Shinada Substory "A Precious Meal")
  
• Mystery Stone - Coliseum Item, (Hostess Gacha Item, various other sources)

Store

• Battery - Bought at Don Quijote in Kamurocho, (Sotenbori)
  
• Kamaitachi Utility Knife - Bought in Ebisu Pawn Kamurocho (Tsukimino, Kineicho) (The Cost of a Swing)
  
• Chili Pepper - Bought in Don Quijote in Kamurocho, (Sotenbori), (Garbage Collection)

Locker Keys

• SSS Gunpowder -Kamurocho Locker A2, (Coliseum Item)
  
• Fitness Gear - Kamurocho Locker B1, (Akiyama Substory "Haggle Wars")
  
• Sturdy Iron Pipe -H1 Locker Kamurocho (Also from Shinadas story.)

Sea Fishing: The Pier

• Maguro - Otherwise known as a Tuna, Tokyo Sea Fishing
  
• Squid - Sea Fishing, (Sotenbori Locker C3)