Guxt

Shield powerup:
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After beating the bosses on stages 1-3, continue shooting at the
eyes to reveal hidden shield powerups! Each eye contains such a 
powerup. Remember that the boss can't kill you after it has been
defeated, so don't be afraid of it!

Cave Story cameo:
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In Stage 4, when there's a dotted line made out of land blocks, fly
over the rightmost block to reveal a pair of glasses. Shoot at them
to reveal Jack from Cave Story.