1-8 (Gear Ring*), Enter (Confirm / Talk), and Escape (Options) are reserved keys, therefore cannot be manually bound to any action.
*more under Basics


As someone who has dedicated at least 325 hours to the Switch release overall and about 10 hours to the Steam version for a segmented Zero Trial run with Dark Wings, I have concluded that these controls suit me best.
(If you know EchecCritiqueMisc on YouTube or Vouivre Critique elsewhere, I am he.)
And, if you're curious about the full thought process behind this control scheme, allow me to shamelessly promote my January 2021 blog post.[vouivreview.wordpress.com]
I prefer to avoid analog crossbow aiming if I can help it (as I sometimes find myself prone to stick drift when using the Joycons), but I have the left and right mouse buttons bound to Tool and Alt Tool in case I ever need to use the crossbow that way.

Blackthifer is a Discord community member who, unlike me, did not play the Switch release and does use the mouse while playing. Since they shared their key binds, I figured I would offer a different perspective in them.

Up, Down, Left, and Right are used to move the cursor.
Confirm / Talk to advance, Cancel / Back to...well, go back.
In the Items menu, Alt Tool is used to "grab" an item, allowing it to be stacked onto a matching item or swapped with a non-matching item. Number keys 1-8, to add items to the Gear Ring. Sprint or Prev Item and Crouch or Next Item, to scroll through the menus: Badges on the left (former two), Items in the center, and Quests on the right (latter two).
In dialogue, alternate between Confirm / Talk and Cancel / Back for optimal text speed, unless you hit a decision prompt and know that you don't want the first option (e.g. sleeping in a bed, where "Yes" is always the second option), in which case it is advised to prioritize Cancel / Back as a means of advancing through dialogue while never being responsible for decision selection.

Use Left and Right to walk, and Jump to...well, jump. The latter also wakes Gail when she rests.
Crouch does what it says on the tin, and allows Gail to change direction without moving a single pixel. (Down also works this way.)
That is, unless you hold down Left or Right, which causes her to crawl to the avail of moving as slowly as possible while having as small a vertical hitbox as possible.
(Up, Down, Left, and Right are also used to respond to quicktime events as they arise, namely in cooking and being immobilized.)
Sprint is primarily used to make Gail move faster at the cost of energy—whether in ordinary sprinting on the ground, rolling while crouched or crawling, or using Jump to leap. (The latter is usually the most energy-efficient of these. While doing the former, abruptly changing direction will cause Gail to skid.) Other uses are described as Advanced Techniques.
Lastly, number keys 1-8 serve as efficient means to equip items in the Gear Ring (where the slots are denoted by Roman numerals), while the Prev/Next Item actions cycle (counter)clockwise through the Gear Ring. (When one item is equipped this way, the other is automatically put away if applicable.)

Gail can interact with...

• people and important objects using Confirm / Talk.
  
• background ladders and vines when Up is used to climb. This is slower than jumping, so do it as seldom as possible.
  
• boxes and similar, primarily through use of the Grab action. While grabbing an object, use Left and Right to push it around, or Up to lift it. Objects no more than half the height of Gail can be lifted over her head and carried around, in which state Grab puts it down and Attack throws it, the latter causing most pots and wooden boxes to break. (This state also makes it easy to jump up steep slopes, e.g. the metal trapezoids in the White Towers.)
  
• water, but not in the most elegant manner. The height from which Gail falls into the water is directly proportional to the depth she dives into the water. However, she will drown and lose horizontal momentum unless she has the Lifesaver, which is described under Items and Equipment.
  
• cats when the Grab action is used on top of them, making them meow and roll over. Once is enough to count towards the Cat Petter badge.
  
• enemies using her bat and/or damage-dealing tools. The former case involves three uses of the Attack action: rapid ground attacks, efficient midair attacks, and staggering charged attacks. (Hold Attack on the ground to charge.) As for the tools, those are described under Items and Equipment.

In the vast world of Phoenotopia, Gail will have to peruse the overworld to find locations beyond the 2D screens of the game.
This overworld presents itself as a top-down environment navigable by the Up, Down, Left, and Right actions. Confirm / Talk at the entrance of a location to...well, enter it. You can also Sprint for faster movement at the cost of energy, but not as fancily as in 2D environments.
Encounter symbols appear based on the number of steps Gail has taken: mostly gray blobs that move in four directions, and sometimes red bats that home in on Gail. Do take care to avoid these symbols unless you...

• need any of the six moonstones exclusive to particular encounter locations: east of Sunflower Road, anywhere in the Sand Drifts region, west of the Antique Shop in Daea, north of the Forgotten Pier in Cosette, south of the Last Wall, and beyond the Last Wall.
  
• seek the Boar Boy southeast of Atai City.
  
• require the Overworld Encounters badge.
  
• are driven by curiosity and/or dubious desire.

If you get sent flying by a source of knockback, you can use the Sprint action before hitting a wall or floor to allow Gail to regain control and not take any damage that would result from the knockback (e.g. when a Wrecker throws you).

When you fall from a high height, or land prematurely during a midair attack, Gail goes through landing lag. This can be canceled through use of the Sprint action, which instead makes her roll without using up any energy.
The end of a rolling animation is similar to landing lag, except canceling it with Sprint simply causes Gail to instantly get back up.

(Credit to Whiskpl, one of the speedrunning greats of the series)
Tumbling is the fastest form of energy-free movement: using a perfectly timed midair attack to generate landing lag, canceling the landing lag with a roll, and then canceling the end lag of the roll.
Somersaulting is a combination of leaping and tumbling, the perfect blend of momentum and efficiency.
Best practice is to somersault whenever possible, and tumble if you need your energy to regenerate.

Press the Tool key to commence consumption, and continue pressing it to speed up the process.
Crouch, or use the Attack or Jump key, to put away the consumable. (The crouching occurs, whereas the latter two actions do not.)

Use the Tool key to pull it out, spawning a flashing reticle that gradually changes in trajectory from (roughly) 135° to 30° (starting upward and rotating in the direction Gail is facing) and back.
(Note that you cannot pull out either of these tools in midair.)
With either tool primed:

• Use the Tool or Attack key to fire.
  
• Press the Alt Tool key to hold the reticle in place, and again to keep it going.
  
• Crouch, equip a different item, or press the Jump key to put the tool away. (In the latter case, the jump does not occur.)

Prior to pulling out either tool, hold Up to make the reticle start at 30° instead of 135°.
The Treble Shot is a three-way projectile that deals 2 extra damage per round yet costs more energy.

On the ground, hold the Tool key to prime the crossbow, spawning a reticle that starts right in front of Gail and stays still unless aimed.
With the crossbow primed:

• Release the Tool key to fire. (The Attack key does nothing.)
  
• Use the movement keys to aim. You can normally only aim it in eight directions, but the mouse or the Sprint key allows for analog aiming. In the latter case, Right moves the reticle clockwise, Left counterclockwise, Up towards Gail's head, and Down towards Gail's feet.
  
• Equip a different item or press the Jump key to put it away. (In the Civilian case, the Alt Tool key also works. The Crouch key, however, has no effect.)

If used in midair, the crossbow will simply fire forward, no matter how long you hold the Tool key.
The Double Crossbow fires twice in a row for 1 extra damage per round yet costs more energy.
It also has an Alt Tool function to switch between spread rounds and split rounds. (The latter makes the rounds deviate (roughly) 10° in opposite directions.)

Press the Tool key to take out the lamp, and continue pressing the Tool key to crank it and increase its illumination range.
Cranking the Refurbished Crank Lamp will occasionally produce sparks that deal exactly 1 fire damage each.
The Neutron Lamp boasts the longest light duration by far.
Use the Alt Tool key to turn off the lamp, and put it away if you have it out.
Crouch, change equipment, or use the Attack or Jump key to put the lamp away without turning it off.

Use the Tool key to cast the line. The longer the key is held, the farther the line goes.
While the bait is out, use the Tool key to reel it in, or the Alt Tool key to make it spin.
When a fish bites the line, it's a battle between energy meters, which presents itself in a mini-game where you line up a needle with a moving ring portion.
Adjust the needle using the movement keys or the mouse. In the former case, Right and Down make the needle go clockwise, while Left and Up make it go counterclockwise. Vertical movement is fast, whereas horizontal movement is precise and can be made fast using the Sprint key.
Barring the "higher-level fish won't invert controls" Accessibility Option, anything with an equal or greater challenge level than a gourmet fish will occasionally invert the ring, making the movement keys behave the opposite of the way they normally do.
If Gail's energy runs out first, then she automatically puts the rod away. Else, the fish is caught.
At any point, press the Jump key or equip a different item to put the rod away.
The Serpent Rod slows the loss of energy during the mini-game.

Use the Tool key to pull it out, and the Jump key to put it away.
(Note that it cannot be pulled out in the middle of a leap.)
While holding an instrument, use the Tool key to play a note, the movement keys to control the note played, and the Sprint key to alter the pitch.
You can learn one of six particular songs by playing it when it is being taught to you. Learning a song means that you can play it at any time, potentially to some special effect.
Non-particular songs are hidden in puzzles throughout the world, to be played for Song Stones guarding treasures of various significance.
The Spheralis serves two additional purposes: to disable Megaliths by going through seven iterations of playing the note displayed on its pillar, and to operate mechanisms indicated by glowing D-pad-shaped engravings in the wall.

Use the Tool key (or Alt Tool key in the non-Remote case) to pull out a bomb.
This bomb qualifies as an object that can be lifted over Gail's head, so it can be put down, picked up, and thrown around at will.
(It also makes for a good stepping stool when needed.)
Only one Remote Bomb and/or two bombs total can occupy the same area as Gail.
Standard Bombs have a 4-second fuse, while the Remote Bomb can be detonated manually with the Alt Tool key.
Be warned that half of their damage output, rounded up, is dealt to Gail if she gets caught up in the blast. (On the flip side, their knockback can prove useful in reaching normally unattainable heights, or using crawl storage to bypass a box-height tunnel, e.g. the path beyond the South Castella Bridge Energy Gem.)

This piece of equipment allows Gail to explore water-filled areas with minimal drowning.
Just hop into a body of water, and Gail will automatically don the Lifesaver to make herself gravitate to the surface.
Your movement is slowed at the water's surface, but you can use the Sprint key to propel yourself forward. This animation has a bit of end lag that can be canceled with a jump, albeit at a significant energy cost.
However, should Gail remain underwater when her energy hits absolute zero (i.e. when every bit of green is gone from the meter), she will drown.

Press the Tool key to prime the spear, and release to throw.
With the spear primed, use Left or Right to change directions.
Use the Jump key, or equip a different item, to cancel a primed spear.
When thrown, spears stick to most walls and serve as extra platforms, but only two at a time can be active.
Note that a ground-level spear toss is exactly one block high, which is helpful for small height adjustments, e.g. on the way to the Lunar Goblet.
The Alt Tool key serves to dismiss all active spears, making explode those that are charged with ki.
(Dismissing a spear while rolling on it is a safer method of storage to bypass box-height tunnels, e.g. on the first floor of the White Towers.)

In midair, use the Jump key to hover with this piece of equipment. The thrusters deal 10 fire damage.
Hovering during a leap will get you slightly more height than a full jump, but hovering after a standard jump is more energy-efficient.

A delayed and high-recoil projectile with two modes: explosive and five-way.
Explosive rounds pack a wallop and...well, are explosive. Perfect for Dark Gail and Katash 2.
The five-way deals more damage at point blank range to enemies with adequately low defense (and works wonders against Mother Computer's barrier), and can serve as a means of chip damage against flying enemies.
Use Tool to fire, and Alt Tool to change modes.

Use the Tool key to make Gail's next attack charged with no windup, or the Alt Tool key to restore Gail's energy.
Caveat emptor: Each function costs up to 4 HP.
(Also, this is the only item whose Tool function can be used while Gail is crouching.)