Who could forget this dialogue from Kid when you meet her in Termina after recruiting Leena and staying at the inn?
Kid: But geez, to go straight to the inn with a young girl...You're pretty slick for such an innocent-lookin' boy!
Or the bit in the Shadow Forest where Nikki asks if you want to hear him sing, and all three options you can choose from are ways to tell him no.
In one of the battles with Solt and Peppor the former does something so stupid that for the rest of the battle Peppor does nothing but attack him. What had he done? HE WASTED A BLACKHOLE ELEMENT(Which could not only instantly kill Serge, but also the entire party).
Luccia tells you not to release Pip...right after you were already given the option of setting him free, and most likely did so.
Upon meeting Norris for the first time and explaining that "this is not Lynx; he has the body of Lynx, but his soul is Serge's":
Any time you meet Harle or she's in your party, though both her first meeting with Kid in Viper Manor and their second one at Hermit's Hideaway (if you save Kid) are especially hilarious. Especially since Harle is essentially the Dragon Gods' copy of Schala, meaning she's bitch-slapping and fighting with herself over Serge.
A few of the endings you get in a New Game Plus:
Beat the final boss with Poshul at the start of the game, and Kid takes over the world, establishing the Acacian Empire.
Beat the final boss after completing Dario's sidquest and Serge is reborn. Dark Serge/Lynx, Harle and Kid go to Termina's bar for a long drinking session...which is run by Luccia, Karsh and Marcy. Much hilarity ensues, one of which being Dark Serge bitching on how he hijacked a body that isn't of legal drinking age, and Harle ordering heavily-sugared coffee.
Beat the game after Viper Manor and choosing not to save Kid results in a terribly harsh but hilarious bit where Serge is now working at Lisa's Element shop and she and Leena proceed to have a catty exchange of cruel insults, she and Kid have a shorter one, and then Korcha tries to romance Kid again only for her to sneak out behind his back and be replaced by his mother. Then Solt and Peppor assault Fort Dragonia...with Pierre whom they've hired based on his false hero cred. He pleads sickness from eating too much at Zappa's, asking them "not to run", then when they get to the top of the fort fakes sickness again (the same way he did when trying to break in Viper Manor's gates) so as to get out of being the one to open the doors. (When the Shaker brothers say they'll try "Rock Paper Scissors" again he immediately leaps up just fine...) After this the screen goes black with the message "The three of them were never seen again."
If you choose to save Kid when she is poisoned, and reach the part where Korcha proposes to Kid (already a CMOF in itself), Serge can choose to encourage or discourage Kid to accept. No matter the choice, Kid will kick Serge in the groin.