Killin can be acquired by playing through the first half of tournament mode; the 2nd character, Nemo, can be acquired by playing through the second half of tournament mode.

Beat all eight tournaments to unlock the Real difficulty.

Unlock the following character's endings by finishing all 8 tournaments, and then completing any tournament with that same character:

ENDING - HOW TO UNLOCK
Adjo Ending - After finishing all 8 tournaments, complete any tournament as Adjo.
Dragov Ending - After finishing all 8 tournaments, complete any tournament as Dragov.
Finn Ending - After finishing all 8 tournaments, complete any tournament as Finn.
Killen Ending - After finishing all 8 tournaments, complete any tournament as Killen.
Kuro Ending - After finishing all 8 tournaments, complete any tournament as Kuro.
Nemo Ending - After finishing all 8 tournaments, complete any tournament as Nemo.
Renwick Ending - After finishing all 8 tournaments, complete any tournament as Renwick.
Seager Ending - After finishing all 8 tournaments, complete any tournament as Seager.

Unlock the following concept art by performing the corresponding tasks:

CONCEPT ART - HOW TO UNLOCK
Bunker Concept Art - Shoot all secret skullshots in all 6 bunker stages.
Manor House Concept Art - Shoot all secret skullshots in all 6 manor house stages.
Ocean Liner Concept Art - Shoot all secret skullshots in all 6 ocean liner stages.
Prison Cells Concept Art - Shoot all secret skullshots in all 6 prison cells stages.
Steel Mill Concept Art - Shoot all secret skullshots in all 6 steel mill stages.
Venice Concept Art - Shoot all secret skullshots in all 7 venice stages.
Warehouse Concept Art - Shoot all secret skullshots in all 6 warehouse stages.
Warzone Concept Art - Shoot all secret skullshots in all 6 warzone stages.

ACHIEVEMENT - HOW TO UNLOCK
Sharp Steel (10 points) - Beat the Steel Mill in the Tournament.
Hard Steel (20 points) - Beat Steel Mill on Insane level.
Splash Down (10 points) - Beat Ocean Liner in the Tournament.
Watery Grave (20 points) - Beat Ocean Liner on Insane level.
Out on Probation (10 points) - Beat Prison in the Tournament.
Hard Time (20 points) - Beat Prison on Insane level.
Waterway to Go (10 points) - Beat Venice in the Tournament.
Wet and Wild (20 points) - Beat Venice on Insane level.
Crate Work (10 points) - Beat Warehouse in the Tournament.
A Crate Effort (20 points) - Beat Warehouse on Insane level.
A Stately Achievement (10 points) - Beat Manor House in the Tournament.
Full House (20 points) - Beat Manor House on Insane level.
Defcon 3 (10 points) - Beat Bunker in the Tournament.
Bunker Buster (20 points) - Beat Bunker on Insane level.
Warlord (30 points) - Beat the game in the Tournament.
Skillzone (40 points) - Beat Warzone on Insane difficulty
Join the Club (40 points) - Play an event with every one of the six unlocked characters in either Tournament or Single Event.
The Awards Show (25 points) - Achieve one of every "in game award" either in Tournament or Single Event.
Show Off (20 points) - Achieve every "in game award" in one level in either Tournament or Single Event.
Save our Souls (20 points) - Spell the word SOS in the Warehouse in either Tournament or Single Event.
Listmania (5 points) - Save 5 Playlists in GunPlay.
Premium Membership (20 points) - Finish a Tournament on any difficulty with all 8 characters.
Rack em Up (10 points) - Achieve your first x10 combo in either Tournament or Single Event.
Hidden Treasures (30 points) - Shoot All Secret Skullshots in every level in either Tournament or Single Event.
Scavenger (10 points) - Shoot All Skullshots on one level in either Tournament or Single Event.
Bag them Bones (30 points) - Shoot All Skullshots on every level in either Tournament or Single Event.
Melee the Elephant (25 points) - Kill 30 enemies in a level with a melee attack in either Tournament or Single Event.
Combo Killer (30 points) - Reach the level objective with a maximum combo in either Tournament or Single Event.
Efficiency (30 points) - Kill every enemy on a level using less than 50 bullets in either Tournament or Single Event.
Breakthrough (10 points) - Smash 50 Doors in either Tournament or Single Event.
Scores on the Board (10 points) - Get in the top 100,000 on any leaderboard.
Tabletopper (25 points) - Get in the top 5,000 on any leaderboard in the Tournament.
Top Gun (25 points) - Get more than 10,000 points for a single kill in either Tournament or Single Event.
Sprint the Gauntlet (20 points) - Beat a Run the Gauntlet with 20 seconds left.
SkullMaster (25 points) - Hit 5 Skullshots in a row (without killing a bad guy in between).
Welcome to The Club (10 points) - Win your first ranked match.
Taking part that counts (10 points) - Be on the winning team in a ranked team game.
Capture King (20 points) - Capture the base 25 times in Ranked matches.
Club on Tour
(25 points) - Win a ranked match in every location.
First Blood (20 points) - 25 "First Kills" in any Ranked Multiplayer game mode.
Big Game Hunted (20 points) - Survive for 120 seconds as the hunted in Hunter Hunted.
Target Man (20 points) - Shoot 100 Skull shot targets
in Team Skullshot.
Fox Hunter (20 points) - Kill the Enemy Fox 25 times in Ranked Matches.
Points Mean Prizes (15 points) - Score 10,000 points in a Score Match.
Free For All Elite (30 points) - Win any free-for-all game without dying.
Siege Breaker (20 points) - Your team survives for more than 2 Minutes in Team Siege.
Demolition Man (20 points) - Get 100 Explosive Kills in Ranked Matches.
One Shot (20 points) - Get 100 Sniper Kills in Ranked Matches.
Brawler (20 points) - Get 100 Melee Kills in Ranked Matches.
No, Seriously (40 points) - Kill 10,001 people in multiplayer Ranked Matches.

Unlock the following character's concept art by performing the corresponding tasks:

CONCEPT ART - HOW TO UNLOCK
Adjo Concept Art - After finishing all 8 tournaments, complete any tournament as Adjo.
Dragov Concept Art - After finishing all 8 tournaments, complete any tournament as Dragov.
Finn Concept Art - After finishing all 8 tournaments, complete any tournament as Finn.
Killen Concept Art - After finishing all 8 tournaments, complete any tournament as Killen.
Kuro Concept Art - After finishing all 8 tournaments, complete any tournament as Kuro.
Nemo Concept Art - After finishing all 8 tournaments, complete any tournament as Nemo.
Renwick Concept Art - After finishing all 8 tournaments, complete any tournament as Renwick.
Seager Concept Art - After finishing all 8 tournaments, complete any tournament as Seager.

Unlock the following levels by completing the corresponding tournaments:

LEVEL - HOW TO UNLOCK
Bunker - Complete manor house tournament.
Manor House - Complete warehouse tournament.
Ocean Liner - Complete prison cells tournament,
Prison Cells - Complete venice tournament.
Venice - Complete steel mill tournament.
Warehouse - Complete ocean liner tournament.
Warzone - Complete bunker tournament.