Big "NEVER!": Leatherhead to the Kraang when they demand the power cell.
Booby Trap: Leatherhead made sure the power cell was very well protected when he hid it.
Both Sides Have a Point: The Turtles get into an argument over how to deal with Leatherhead; Mikey believes that he deserves their sympathy, but the others don't want him around because of his instability and tendency to go into Unstoppable Rages. Splinter compromises; Leatherhead should be shown compassion, but he should also be kept chained up so he won't hurt anyone.
Butt-Monkey: This episode takes this to truly ridiculous levels, wherein Don gets thrown around like a rag doll, shoved into the water multiple times, shocked by high-voltage technology, and then volunteered to face Leatherhead (the one who threw him around in the first place). This is allPlayed for Laughs, despite the fact that he is clearly terrified by the last part.
Dark and Troubled Past: Leatherhead's backstory. As a baby, Leatherhead was a boy's pet, but his parents flushed him down the toilet when they discovered him. He was later caught by the Kraang, mutated, and experimented on.
To elaborate, the turtles (excluding Michelangelo) and Splinter were reluctant to let Leatherhead stay in the lair. Mikey points out that they were doing to Leatherhead what everyone else did to them, judging based on looks.
Expressive Mask: More so than usual. When yelling at Mikey, poor Donnie gets so agitated by his mistreatment that even the tails of his mask stand upright in indignation.
Extreme Omnivore: Mikey makes a pizza-milkshake combo and calls it a "P-Shake". He takes a swig, spits it out in disgust... and then drinks the rest of it anyway.
Leatherhead likes Mikey's pizza noodle soup, and Mikey is surprised that somebody finally likes his cooking. How's that for extreme?
Given that Leatherhead had been trapped with the Kraang for several years, anything tastes better than what they probably fed him.
Splinter (about Leatherhead): His scars run deep, but they protect a genuine heart; When he awakens, Michelangelo, you must find a way to befriend him. Donatello: But we're still chaining him up, right? Splinter: Of course! I am compassionate, not insane!
Hypocritical Humor: In Space Heroes. Captain Ryan says "If there's one thing I'm known for, it's my humanity." Yet his idea of humanely getting rid of the annoying little Trumpet aliens is to suck them out the ship's airlock.
Monster Roommate: Leatherhead. Not that the Turtles are all that normal themselves, but by comparison...
Neck Lift: More like Head Lift. Leatherhead's hands are so huge he can grab Donnie's entire head and lift him off the ground with it.
Overdrive: What happens when Donnie hooks the power cell up to the motor of a subway car.
Papa Wolf: Later in the episode when Leatherhed loses it and fights the Turtles for the power cell, Splinter comes to their rescue and wins.
Splinter: Get away from my sons!
Plot Coupon: Leatherhead knows about a "power cell" that the Kraang are looking for. Naturally the Turtles have to keep them from finding it. It turns out the Kraang need it to open the portal between their dimension and ours.
Reasonable Authority Figure: After Splinter hears about Leatherhead's problems with the Kraang, he allows Mikey to let him stay in the lair, but only as long as he's chained up.
Splinter: I am compassionate, not insane.
Reluctant Psycho: Leatherhead is fully aware of how unstable he is and isn't happy about it.
Running Gag: Donatello having things latch onto his face.
Shown Their Work: Leatherhead uses several maneuvers real crocodilians use to hunt with when he fights, such as vigorously shaking prey to tear it apart and the famous Death Roll (which he uses to crush Raphael against and grind into the floor), and a powerful Tail Slap.