After completing the game, either S rank 5 episodes or just complete 50 episodes to unlock the hidden episode containing the true ending.

Unlock the following Gauges by performing the corresponding tasks:

GAUGE - HOW TO UNLOCK
Augur - Complete all episodes with the Blind Master Gauge equipped.
Blind Master - Achieve a Total Rank of "A" or higher for all episodes.
Defender - Achieve a Total Rank of "A" in three episodes in Part I: Suffering.
Destructor - Achieve a Total Rank of "A" in three episodes in Part II: Rebirth.
Mortal - Achieve a Total Rank of "S" in ten episodes.
Obliterator - Complete all episodes with the Mortal Gauge equipped.
Survivor - Achieve a Total Rank of "A" in ten episodes.
Thermoregulator - Achieve a Total Rank of "A" in three episodes in Part III: Karma.

ACHIEVEMENT - HOW TO UNLOCK
A Cry of Anger (15) - Activate Burst a certain number of times.
A Roar of Fury (20) - Activate Burst a certain number of times.
Anger Management (15) - Activate Burst for the first time in Episode 1.
Art Appreciator (15) - Get all the illustrations.
Be the Fist (20) - Achieve a certain percentage for your overall synchronic rate average.
Can't Touch That (20) - Perform a certain number of counterattacks.
Can't Touch This (15) - Perform a certain number of counterattacks.
CG Collector (15) - Get all the CG art.
Deity (90) - Complete all episodes with an S rank on Hard.
Demigod (30) - Complete all episodes with at least an A rank on Normal.
Denied (15) - Get scolded by your former master in Episode 10.
Enlightenment (0) - Unlock all ASURA'S WRATH achievements.
Fighter (30) - Complete all episodes on Normal or higher.
Guardian General (30) - Complete all episodes with at least an A rank on Hard.
Heads Will Roll (15) - Completely lose it in Episode 12.
Hit'Em Hard (15) - Defeat a certain number of enemies with a special attack.
Hit'Em Harder (20) - Defeat a certain number of enemies with a special attack.
It Takes Two (30) - Defeat Deus with Yasha in Episode 17.
Ka-ching! (30) - Accumulate a certain number of battle points.
Karma (10) - Complete Part III: Karma.
Lightning Reflexes (20) - Achieve a certain number of EXCELLENT synchronic impact rankings.
Like a Fish (15) - Knock back a few bottles.
Look Ma, No Eyes! (30) - Complete all episodes with a certain gauge equipped.
Mortal (15) - Achieve a D rank or higher on any episode.
Movie Buff (15) - Get all the movies and interludes.
Pain Is Universal (15) - Prove that you can take a beating.
Quick on the Draw (15) - Achieve a certain number of EXCELLENT synchronic impact rankings.
Rebel with a Cause (15) - Do what you think is right in Episode 13.
Rebirth (10) - Complete Part II: Rebirth.
Shinkoku Soldier (30) - Complete all episodes with an S rank on Normal.
Shut Up, Augus! (15) - Interrupt your former master's monologue.
Shut Up, Kalrow! (15) - Interrupt the old one's monologue.
Shut Up, Wyzen! (15) - Interrupt the fat one's monologue.
Sometimes I Feel Like... (15) - Find all the peeping toms at the hot spring.
Suffering (10) - Complete Part I: Suffering.
Taking Out the Trash (15) - Defeat a certain number of enemies with heavy attacks, lock-on fire, or both.
The Six-Armed Man (15) - Grow a few extra limbs in Episode 5.
The Truth Hurts (10) - Uncover the truth.
The Whole Nine (15) - Get all the gauges.
They Don't Stand a Chance (15) - Defeat a certain number of enemies while in Unlimited Mode.
Too Legit to Quit (15) - If at first you don't succeed...
Unstoppable Force (20) - Defeat a certain number of enemies while in Unlimited Mode.
View of the Valley (15) - Give in to your male instincts.
Warrior (90) - Complete all episodes on Hard.
Who Needs Health? (90) - Complete all episodes with a certain gauge equipped.

"Episode Pack: Part IV" achievements
ACHIEVEMENT - HOW TO UNLOCK
Gaea's Champion (90 points) - Complete all episodes in Part IV with an S rank on Hard.
Daddy's Angry (15 points) - Defy the supreme being in Episode 19.
A Friend in Need (15 points) - Protect your brother-in-law in Episode 20.
How Do Ya Like Me Now? (15 points) - Receive a parting gift in Episode 21.
Blaze of Glory (15 points) - Put an end to your wrath once and for all.
Nirvana (15 points) - Complete Part IV - Nirvana.

"Lost Episode 1" achievements
ACHIEVEMENT - HOW TO UNLOCK
A Rebuttal of Fists (15 points) - Complete all missions in Lost Episode 1.

"Lost Episode 2" achievements
ACHIEVEMENT - HOW TO UNLOCK
An Everlasting Clash (15 points) - Complete all missions in Lost Episode 2.