TWilight INSanity Mechanics
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Infection Rate:
Begins at 50%. Increases as long as enemies are on the screen. If it reaches 100%, it's an instant game over (Continuing forces you into a Bad Ending). Infection Rate has a major effect on plot events and available choices.


Ratio of how much each partner has been used. Also has an effect on plot events. Certain events will prevent switching characters but usually you can switch at will.


The multiplier is affected by how fast you defeat viruses. The maximum is 4x. This also has a direct effect on eCrystal generation (see below).


Upper Left: eCrystal
Generated when enemies are killed. Automatically collected. Reduce the Infection Rate. Number generated depends on how fast enemy was killed.
Upper Right: Score Crystal
Increases score. Each one is worth more than the previous (20, 40, 80, etc up to a max of 40960)
Lower Left: 1Up
Obvious
Lower Right: xCrystal
Increases Infection Rate

Tension:
Up to 9 levels can be stored. Using Tension converts all bullets currently on the screen into eCrystals, automatically collects all items on screen, and increases shot power for a few seconds. You are NOT invincible while Tension is in effect.

Auto-Tension:
Automatically occurs if you get hit when you have at least 1 Tension remaining. Uses your Tension attack, reduces Tension stock to 0, and increases the Infection Rate by 5.


Enemy Tension:
Berserk/Last Spell mode basically. All bosses and some midbosses have this. Beating it quickly earns a Tension Break bonus to the score and reduces the Infection Rate.

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Routes go from A to C. You can be booted down from C to B or B to A if the Infection Rate goes above 40 or if the Twins Rate goes above 80. Choices made invisibly affect the Infection Rate for ROUTE PURPOSES ONLY.

The choice made at the end of stage 3 determines whether you go onto C, B, or A, but not all choices will be available depending on previous choices made and the Infection Rate. You can only move downward once you're on a route, never upward.

True End (Stage 6C/7) special mechanics

Ending goes to whoever has the higher Twins Rate.
For the Sequel/Ex endings, the second choice must be made when the option comes up as well as Infection Rate < 50%, Twins Rate < 80% when defeated.
Note that the Ex boss has a special variant of her final attack pattern if Infection Rate < 30%
Bad ends have no CGs associated with them

Ending List


More info on choices/stuff
http://infinite.xrea.jp/etc/twins/

TWilight refrAIN mechanics

Everything for TWINS holds plus:

Shared Life/Bomb System
Tension and lives are replaced by Spirit, which is your life and bombs. The exact cost of auto-bombing and normal bombing depends on the style you choose at the start of the game. It's game over if you're hit and do not have enough Spirit left to auto-bomb. Spirit is gained by collecting Crystals (replace eCrystals in TWINS) that are generated by defeating enemies. See also below. Spirit is also rewarded between stages based on Buzz (graze) rate.

Additional Crystal Multiplier
There is an additional multiplier for crystals off enemies up to 2 for proximity to enemy when you defeat them. IE, defeating an enemy quickly and from close to them gives a multiplier of x6 instead of x4. Bosses (including midbosses) have that increased by 4, so a x8 multiplier is possible if you defeat them quickly. Bosses also have a magic circle showing the minimum range for the +4 bonus. All that matters is the final hit for determining distance.

Adaptive Difficulty and Scoring
The difficulty chosen at the start will fluctuate during the game. It increases through the same mechanics as the P-Rate decreases and decreases very slightly when bombing and significantly when auto-bombing. The amount it fluctuates depends on the initial difficulty chosen. The higher, the quicker it changes. The value of score items is directly related to the current difficulty. S+ is the maximum difficulty/scoring if initial C or B difficulty is chosen. SS is the maximum if A is chosen. Point Items begin at 1600 points (rank D) and double for each letter increase above that (no diff between B and B+).

Secret Magic
Every boss and midboss has a secret variation of their final attack (or for most midbosses, an additional attack) for fulfilling certain conditions during the stage. You also cannot autobomb against the boss (I think). The exact number varies by the current difficulty. Mid bosses and bosses on the same stage share the same requirements, but at different levels.

Secret Magic Triggers by stage

NOTE:
To get the good ending for the Sequel/Extra stage, you must trigger the Secret (Primal) Magic for the boss.

Rings
Granted by choices and gameplay, and are the sole deciding factor for which route you're on. The number of rings rewarded for a story choice are shown after the choice is made and then displayed for all future playthroughs. A low P-Rate rewards you with extra rings at the end of a stage. A high P-Rate or high Twins Rate penalizes them. <40 needed for stage 1, <32 for 2, <20 for 3, <10 for 4 and 5

Stage Split
In addition to the split of three variations of stage 4, 5, and 6 (E, F, and G) that's the same as in TWINS, the progression of stages 2 and 3 is different. If you do well in stage 1 (A) (>= 2 Rings), you will proceed to stage C1 and then D before E onward. If you do poorly in stage 2, you will proceed to stage B and then C2 before E.

Full Stage branching list by rings


Stage 7 Mechanics