Ever wanted to beat your friends to death using their own cards? Then Sumika is the perfect character for you!
Sumika can copy (almost) any card and use (almost) any of them for free! Her stats also allow you to beat your friends to death even if you don't get to copy their cards! Sumika's capable of anything!
This guide will go into (way too many) details on how her hyper works, and teach you how to always make the right decision and calculate your way to victory as Sumika.

Sumika is a very versatile character, with the same stats as Hime and Mira, but without any passive. She has no trouble getting wins due to +1 ATK, and can be a threat by herself. She can also play passively and survive a few hits due to 5 HP, but risky fights should be avoided due to her below average defense.
Her hyper allows you to copy the 2 most recent cards in the discard pile, and they are treated as having no cost and being Level 1. Although there are a lot of technical things in this effect, once you get used to it, getting the cards you want will be no problem.
As a general gameplan, you should be either trying to collect stars, or bullying weak characters such as Marie Poppo, as well as drawing cards to get your hyper to copy cards in the discard pile.




Pros:

• Versatile
  
• Good ATK and EVD stats
  
• Hyper allows her to use powerful cards

Cons:

• Low DEF
  
• Hyper is situational and complicated
  
• Relies on other players having good cards

Reproduction of Records allows you to copy the two most recent cards in the discard pile, and use they are treated as having no cost and being Level 1.
The discard pile, however, is not the list of the most recent cards, but rather an invisible pile that any card that is from the deck goes after being used or discarded. Cards discarded are sent to the discard pile, even if they don't appear in the list of recent cards.
You should keep track of discarded cards to be able to use Reproduction of Records without failing.
When a card is used, it is first send to the discard pile, and then the effect is applied, so a card like Bad Pudding will make the top card of the discard pile be the card discarded by its effect.
Trap cards are only sent to the discard pile when they are removed from the field, either by being stepped on, or by an effect.
Any cost specified in the effect such as Tactical Retreat will still have to be paid.

What cards can I copy?
You cannot copy Reproduction of Records as it is stated in the effect.
Any effect that creates a new card, such as Extension, will have that new card brought from outside the deck, and therefore removed from the game after used, rather than sent to the discard pile.
Effects that create new cards:

• Branch Expansion Strategy (creates new hyper cards, so the hand is discarded and then hypers are added from outside the deck.)
  
• Do Pirates Fly in the Sky?
  
• Evil Spy Work - Preparation
  
• Extension
  
• Mei's Passive
  
• Minelayer
  
• Miracle Walker (same as Branch Expansion Strategy)
  
• Observer of Eternity
  
• Playground! (Hyper Cards gained through it)
  
• Plushie Master
  
• Reproduction of Records (cards copied by the effect)
  
• Spores (Mushrooms)
  
• Star Blasting Fuse
  
• Treasure (Blue Treasures)
  
• Co-op Sweet Guardian

Effects that modify a card, without creating any new card, such as Yuki (Dangerous)'s passive, transform that card into a new one rather than creating a new card, thus the new card will be copiable by Reproduction of Records.
Effects that transform cards:

• Cards that passively change form while in hand. (Angel Hand/Devil Hand, Magical Revenge/Miracle Red Bean Ice Cream, Leap Through Space/Leap Through Space (Marking), True White Christsmasher/White Christsmasher, Big Rocket Cannon)
  
• Rocket Cannon (active effect to change to Big Rocket Cannon)
  
• Mimic (the type of the hyper is just changed without it being discarded, so no new hypers are created)
  
• Yuki (Dangerous)'s Passive

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Multiple cards discarded at the same time will be discarded from right to left. (small preview of players 2 and 4 are reversed.)
Examples: Princess's Privilege; Flamethrower.

Multiple trap cards being removed from the field at the same time will be sent to the discard pile from most recent set to oldest set. The oldest traps set will be on the top of the discard pile.
Examples: Star-Blasting Light.

If a card states that it goes somewhere else after being used or discarded, it won't go to the discard pile.
Examples: Unlucky Charm; card discarded by Tragedy in the Dead of Night; Leap Through Space (Marking).

Cards that change into another card by a passive effect will not return to its original cost and Level.
Examples: Angel Hand/Devil Hand every odd/even turn; Big Rocket Cannon upon KO or another card being used; Yuki (Dangerous)'s passive.

Cards that change into another card by an active effect will return to its original cost and Level.
Examples: Rocket Cannon's first effect; Mimic transforming Hyper cards.

Cards copied through Reproduction of Records are still treated as their original Level for effects.
Examples: Santa's Job (does not activate effect on a copied card unless you have level equal to or higher than the card's original Level, and gives stars based on original Level.)

Reversed cards show their actual cost and Level to other players, rather than original, so other players can see what cards are copied if your hand is reversed.
Examples: Melting Memories; Sealed Memories.

If Reproduction of Records is one of the two most recent cards in the discard pile, it will be skipped, giving the user the other two most recent cards instead.

The discard pile becomes the deck once the entire deck is drawn, making it empty and leaving no cards for Reproduction of Records to copy until new cards are discarded or used.

If Gift Exchange is used, any card copied by Reproduction of Records will return to their original cost and Level.

Tragedy in the Dead of Night and Treasure Thief will not make a copied card return to its original cost and Level, even if sent to another hand, despite Reproduction of Records' text saying otherwise.

Cards stored by Present Thief are not sent to the discard pile, additionally, if a Present Thief that has been stepped on once leaves the field due to Star-Blasting Light, Stiff Crystal or Crystal Barrier, the Present Thief will be sent to the discard pile first, and then the cards stored by it.

You cannot copy the cards played by the boss in co-op.

[Things yet to test: if cards reset on scrambled eve, sweet destroyer, exchange, present thief; Suguri 46B with copied cards]

Due to her low defenses, you should avoid battles with bullies as they will likely damage you by a significant amount. You can kill characters at low HP easily due to the +1 ATK, and bully characters with low ATK easily as you can evade their attacks. Try to draw hypers early to be able to copy strong cards later. Avoid relying on the hyper card of the player whose turn is right after yours, as there will be two players inbetween them using hyper and you being able to copy it, meaning the players inbetween could use or discard cards to make you unable to copy it. Avoid boss tiles at all costs once it spawns, as it will likely kill you. If other players don't have many cards, and you don't think they will use any, discard a strong card to copy it on your next turn. Keep track of discarded cards to know when to use Reproduction of Records, and calculate your way to victory!

Sumika is very versatile, so you can use pretty much anything you want to use in the deck. She can however, benefit from high cost and high power cards due to her being able to copy them and use them at no cost, so you can use cards such as Accel Hyper in the deck.
She also benefits from cards that allow you to know what someone else discards, such as Passionate Research, which will let you know what someone discarded if they discard the card they just drew.
Some other helpful but not as consistent cards are Brutal Prank and Sealed Memories.
Sumika's capable of anything!

Keep track of the discard pile, fear Nico using Miracle Walker right after someone uses a card you want, and use strong cards for free to gain a big advantage and win the game!


Credits
kuuhaku_naibu (aka Nihil) on testing
Tekkahedron for inspiration
Reiji's Guide for teaching me how to use Reproduction of Records. This guide is also more begineer friendly.
100% Orange Juice Wiki for the images

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I've not been playing the game so this guide may have outdated information, although I don't think anything has changed since. Maybe just some interaction with a newer hyper. Feel free to ask and I'll probably be willing to test if there's something not here.
2020/09/24 - Added Rocket Cannon and Big Rocket Cannon to list of cards that transform.
2020/09/25 - Added interaction with Santa's Job; changed Gift Exchange part, as Tragedy in the Dead of Night doesn't make cards return to original cost and level; added information in the Present Thief part about Present Thief with stored cards being discarded/negated; interaction with reversed cards.
2020/09/28 - Added credits and added a bit more to gameplan.
2020/09/30 - Updated specific ruling section to focus on the ruling rather than specific interactions when possible, and also to look a bit more nicer.
2020/10/05 - Updated Tragedy in the Dead of Night. Added what happens when there is Reproduction of Records in the discard pile. Added information on discard pile reset.