Tear Jerker / Sgt. Frog

• The Season One Finale (and the manga equivalent, "The World's Smallest Invasion") is probably one of the biggest. It would be hard to come up with a more painful way to retouch upon everything shown in prior episodes.
  • Of special note is when Dororo and Koyuki say good-bye. Koyuki was the only one to never mistreat Dororo, something he immensely appreciates. Their farewell hug seals the deal.
  • Even after that, there's the scene where Fuyuki discovers a model kit that Keroro left behind after the platoon left Earth. Fuyuki doesn't have his memories of Keroro any more but upon looking at the box he feels like something's missing and starts crying. This part's such a tearjerker that Funimation's Gag Dub of the series, which otherwise changed the dialogue up to add humor in even in moments that were supposed to be more emotional, completely played this scene straight.
    • Before that, there's a rather tragic Call-Back to the first episode where Fuyuki and Natsumi first met Keroro (with the same music to boot), and Fuyuki suddenly starts running down to the basement that used to be the platoon's base, only to find it the way it was in the first episode. Fuyuki remembers the encounter with the ghost too, implying this helped jog his memory but he still doesn't know what's missing.
• The first movie, all the human/Keronian partners breaking up and everyone's subsequent reactions. *sniffle*
• Keroro and Alternate Universe Fuyuki parting ways in episode 205. A surprisingly, and rather strikingly good, tearjerker moment.
• The ending of episode 262 has the potential to start waterworks too. We have Keroro hurrying to the station to catch his mother before she leaves on her flight home, to apologize to her for hurting her feelings. He doesn't get there in time.
• The Season Two Finale with the Garuru Platoon. The rest of the Platoon's been taken out and Keroro's now Brainwashed and Crazy and ready to take over the Earth. Fuyuki runs up to him and begs for him to stop, screaming for him to come to his senses. It works.
  • Just imagine what must be going on in Fuyuki's head. At the start of the finale, he recently had a quarrel with Keroro for something he isn't responsible for and that was the last time they talked before the real culprits made themselves known. Understandably, Fuyuki broke down into tears after seeing Keroro's current state.
  • Natsumi was not far behind. She was the one who pointed fingers at Keroro only to later learn that despite what happened, he still tried to protect her and her family from the Garuru Platoon. The slap she gave him was out of sheer desperation to bring back the old personality of Keroro.
• Episode 284. Even Keroro's announcement of the title is melancholic. When a new commander comes and put forward an extremely evil plan, Dororo ends up cutting ties with his platoonmates, all while trying to hide this from Koyuki to not worry her. Later, when Keroro and the others are in mortal danger, Dororo initially refuses to save them, being too bitter over what happened. Thank goodness Koyuki's What the Hell, Hero? speech helps him see reason.
• Movie 5 where everyone reacts to Keroro's seeming death, especially Giroro. It gets even worse after Aku-Aku descends to mock Keroro's death in a major Kick the Dog moment, with Giroro screaming to the sky in anger and grief.