SWAP-WORTHY
You can swap anyone or anything in if you just plan to bluff, but there are certain characters and items which what I truly considered "Swap-Worthy," a title I give to certain characters and items that could truly change the cycle of specific situational battles. These are the characters and items that are perfect for Defensive Swapping.
1. SWAP-WORTHY CHARACTERS
(Anti-Drain/Ki/Blood)
Jinsei is an excellent swap whenever you see drainers which mostly have Hana and Amaya. Against Yume however is a different story since she can stun Jinsei's tactic skills. Also if you are facing against a Shoji and simply want a lot of reassurance against him, swap in Jinsei AND Painkiller.
(Melee Control)
You might say Daiki might be a good swap just for his walling capabilities, but his efficiency comes from his zero cooldown melee counter. He is perfect for those Juns, Tadaos, and Takeos. Watch out for stuns though.
(You Can't Run Away)
If you are against certain walling teams and have about zero stunners with them, Tadashi is the man to split open that barricade. If the enemy do have at least some stunners, be ready for some sake.
(King of Afflictions & Nightmare of Archers)
Do you spot a protector spammer? Kamiko would be your answer right? Well she would be good, but she's easily countered by Amaya and heavy attackers. Shoji is great for those SINGLE characters who rely on protectors and DRs. Shoji is also a good pick against those full-ranged teams.
(Anti-Tactic/Ranged/Stun)
Yume is like a Jinsei in terms of swapability, except more. Her affliction increases energy usage but ALSO reduces Ranged Damage. Her Fortune can give unstunnable status and a small defense bonus. Most of you would just swap her in for Waking Dream. It is a devastating move to stun all possible tactical skills. Just don't plan on using her against WoR teams.
(Nightmare Counter and Attrition)
Swapping her is very rare but she works wonders on occasions. If the enemy doesn't have spikers or pure afflictions and you need the extra help in countering killer attacks like Kurai's, Tadashi's, and Kamiko's, Amaya is the girl for these occasions.
(Anti-Drug & Major Slowdown)
Having Jatai in the sidelines is very helpful. She can slow down enemies either by draining or increasing cooldown. A single energy loss per turn can disable your opponent's full potential. Most of all you would swap her in to remove pesky drugs like Painkillers and Sake.
(Everyone's Favorite Tank)
Do you spot the infamous AoE teams in the distance? Takeko will be your main protector. Ootake's high HP and reduction support has been one of the most annoying things AoE teams will encounter. Make sure to bring in a Smokebomb just in case Ootake cannot be summoned early.
(Bane of Skill Spammers)
Nori is quite a good swap against any teams who are reliant on being able to use skill after skill every turn. It would either be aggressive attackers or pure drainers. Once Ookami is summoned, those teams' effectiveness and speed has already been halved.
(Anti-Heal & Unlimited Battery)
I swap Toraji mostly when I see a full Drain team which is full affliction. Just summon Byakko against full drain/affliction teams and earn that 1 extra energy without a care in the world. His Strength of the Tiger King is a bonus to lock down those pesky heals.
(Inescapable Mental and Melee Lockdown)
Niwa is a perfect swap if you want to control 2 characters with lethal melee/mental skills. I mostly use it for those pesky AoE wall spammers like Yuurin and Kurai.
(Burn all Protectors)
There's no point in arguing. Kamiko is the perfect way to fully destroy all protectors of enemies especially if the enemy's full team relies on it.
(You cannot run, you cannot hide)
Yuzuki is a popular choice if the enemy loves to stall with walls, counters, protectors, and DRs. With her combination of formations, the enemies will have no way to defend themselves.
(Plague of Slow Teams)
Jikkouso Wallers may not be enough to handle spikers but her main goal is to slaughter slow, attrition teams who rely on stuns, afflictions, and drains. She can also become quite the god against characters like Akira and Tadashi.
(Item-Stealer & Tactic Killer)
Sainan is the perfect swap if the present enemy team relies heavily on items, and key tactics. An example of such would be the high-tier teams reliant to WoR. I deem her the Anti-Shoji.
(Friendly Skills are banned in my eyes)
Arasoi is the embodiment of anti-wallers or anti-support characters. His simple Smokescreen is enough to shut down Jinsei and Yuurin's support capabilities as an example.
2. SWAP-WORTHY ITEMS
(Nightmare to Preps and Channeling Alike)
This is a perfect item to counter those pesky 1 turn WoR'd preps like Book of Mentality and Ken as well as channeling nightmares like No Tomorrow, Devour, and Leeching Sickle. Just make sure you can defend your Smoke Bomber from permastuns.
(Aggressive Melee Self-Destruction)
A perfect swap for those full melee teams which require momentum or attack often like Jun, Takeo, Jyouzo, etc.
(Potential Ranged Lockdown)
If there are specific characters in an enemy's team which have lethal ranged attacks like Niwa and Hana, swap this in.
(Early Staller)
Sleeping Powder may not be much but if you want a particular mental skill to be stalled like Ryu and Jikkouso in particular, this is a great bet.
(Melee Lockdown)
If your enemy tends to have at least 2 melees who are not very aggressive attackers, this is a good item for a diversion. Even better when you have extra ways of adding protectors to it like Crimson Viper of the Hebi and Kame's Genbu.
(Firefighter)
Kamiko, Saigai, & Youko are present potential Fire AoE. Wash the pain out with this item.
(Affliction-be-gone)
It might not remove Afflictions themselves but they ignore Affliction damage for 4 full turns that they might as well not exist.
(I can't feel the stun)
A very good swap if you feel threatened by potential permastuns or channeling killers.
(WoR's Sweetheart)
If you are threatened in getting your Jinsei locked by Yume, this is a good item to spam during those Waking Dream turns in order for you to do a tactical skill in a crucial moment. There's no cooldown or affliction damage. Just spam it away if your character needs not use any skill.
(It's as if nothing happened)
One of my most famous combos is to spike one character to death while using the flower. IT will intimidate the enemy to see a full HP 3v2 handicap. Either ways this is perfect if enemy is a very aggressive damager like Jun, Jyouzo, Takeo, Asami, Yuudai, etc.
(I countered your counter)
There may be few counters as of now, but they will hurt a lot especially when you waste your high-cost, high-cooldown move just to remove possible counters.
(Bite Me!)
If the enemy has at least 2 melees and you have someone who can summon up some extra protectors, insert the viper to the character BEFORE he summons the protector. This way, the newest protector will defend the Crimson Viper which will be insanely lethal to any melee who would dare destroy it.
FINAL NOTE: I will continue to edit in good insights from other players who post as well as update this guide for every balance update and new characters and items revealed.
BLUFF YOUR WAY TO FAME AND GLORY!