Here is the full list of all 20 YUKI achievements.
Finished Episode 1
Finished Episode 2
Finished Episode 3
Finished Episode 4
Finished Episode 5
Finished Episode 6
Completed the intro level
Completed the game with K1-M1
Completed the game with Tracking Star
Completed the game with X-Ploder
Completed the game with Sonic Slasher
Completed the game with Swarm Bizarre
Play 7 minutes of Endless Mode in 1 run.
Reach Space Lieutenant on Standard Level.
Reach Space Commander on Expert Level.
Reach Elite Space Ranger on Insane Level.
Reach Elite Space Ranger without any health bits.
Reach Elite Space Ranger without any power-ups.
Reach Elite Space Ranger without killing any Yokalien.
Kill 1500 Yokaliens in Endless Mode.