The Ancient One offers to tell Leonardo a story about Master Yoshi.
The rock monster tells Leonardo he knows him better than he knows himself. He's actually a manifestation of his anger and guilt.
Gass Hole: Leonardo's traveling companion, in this episode at least
Genre Savvy: Mikey recognizes that Leo is about to finally "uncork."
Heel Realization: The entire point of this episode is for Leonardo to see what he's become and to heal. The final fight (where he realizes he's fighting his own rage and guilt) is the moment where it finally dawns on him.
Jerkass Has a Point: During a heated training session, Leo says Splinter only teaches him things he already knows. Afterwards, Splinter admits this is somewhat true—believing Leo needs a new master to help him through his inner rage.
My God, What Have I Done?: Leonardo has an example of this when he loses control and injures Master Splinter. Although he doesn't actually say the line, the look on his face after it happens and his subsequent remorse show he was clearly shocked, and deeply regretted his actions.
The Only One: Played for Drama. Flashbacks show the others (reeling from fighting with Shredder) barraging Leo with pleas for help and to come up with a plan.
Donatello: Help me. I'm hurt. Michelangelo: We're not gonna make it. Leo, help me! Raphael: Leo, the Shredder's comin' for us! You gotta do somethin'! Splinter: Save us, Leonardo! We are all counting on you, my son.