Full variable explaination. Useful variables are in bold.
Name	What it does
c.acts	Used in Flowey editor mode for storing some numbers
c.att_param1	Parameter 1 of attack, used to pass data from g_monster to attack sprites
c.created	If it's 1 battle starts, if it's 0 title screen shows.
c.customgameover	g.gameovertext is set to its value when green flag clicked
c.customname	Player's name. If it isn't set to anything, default name is used. Can't be changed in-battle.
c.displaymusic	Displays music name while choosing it.
c.dmg1 and 2	To be honest I don't remember, probably something important.
c.guistate	Keeps track of current gui page in advenced mode.
c.inv	Amount of invicibility frames player has after being hit.
c.mon_hp	g.mon_hp and g.mon_hp.max are set to its value when battle starts
c.music	Current music.
c.numberofattacks	Probably unused?
c.originalcreator	If its value matches your username, EDIT button will show up in battle.
c.pl_hp	g.pl_hp and g.pl_hp.max are set to its value after battle starts
c.randomtext 1 2 3	Random text tgat may appear in the menu.
c.randomtextbubble 1 2 3	Random things monster can say.
c.rtexttoggle	Either "on" or "off" - enables/disables monster saying random things.
c.sID	Sound ID used by monster
c.sparecounter	Uses of ACT left to be able to SPARE monster
c.sparecounterreset	c.sparecounter is set to its value after battle starts.
c.temp	For storing some temporary information.
c.texttosimulate	Used in text preview thing
c.version	Version displayed in the editor menu.
g.addhp	After item is used, it's value is shown as number of HP restored.
g.arenasize	Bullet board's size, either 1 or 2.
g.att_var	Used for attack sprites cloning. Aditionally, if set to end, all clones of attacks will despawn
g.bulletcount	Probably unused?
g.custommusic	Used in setting custom music
g.dmg_%	% of damage to deal. Its value depends on g.dmg_b
g.dmg_b	Its value depends on how far was cursor thing from the center of fight-thing after attacking
g.dmg_deal	Result of damage calculation.
g.dmg_r	Random value from 0 to 2
g.f_state	State of battle.
g.gameover_music	1 - normal music, 2 - dogsong
g.gameovertext	Text that appears on game over screen.
g.i	Used in some things, kind of temporary variable.
g.karma	Current Karma Points, requires g.karmaeffect set to 1 to work.
g.karmaeffect	Either 0 or 1 - Turns karma effect off and on.
g.lv	Player's LOVE. Can't be modified after battle starts (is set in g.undertaleprint sprite)
g.mon_att	Monster's attack stat.
g.mon_def	Monster's defense stat.
g.mon_hp	Monster's current HP.
g.mon_hp.max	Monster's max HP.
g.mon_spareable	If it's 1 monster can be spared, else it can't.
g.pl_att	Player's attack stat
g.pl_def	Player's defense stat.
g.pl_hp	Player's current HP.
g.pl_hp.last	Player's HP on previous frame. Used by g.hpbar_draw sprite.
g.pl_hp.max	Player's maximum HP.
g.pl_wp.att	Player's weapon attack (even though there are no weapons… YET =) )
g.preboot	Used by stuff that will happen before your turn thing.
g.random 1 and 2	Temporary variables.
g.shortusername	Username cut to 6 characters.
g.soul.inv	Current invincibility frames
g.soul_col	Soul's color
g.soul_velx	Unused yet
g.soul_vely	Blue soul's Y velocity
g.turncounter	Current turn in battle. Use it for progressive battles!
g.usedthing	Action made by player this turn
text.actX	Text that appears after using act X+1
text.check	Text that appears after using ACT1
text.clear	Used by my crappy text engine
text.encounter	Text that will appear in the menu next turn.
text.flee	Text after choosing flee
text.mon_text_bubble	An unnecasary variable that overcomplicates monster's random text.
text.spare	Text after SPARing monster
text.win	Text after killing monster