Harle: Ooh la lah... So, you still insist zat you are Serge, oui? Regardless your memoire az zis Serge... You may be able to rewrite it if you use ze Dragon Tear. In realite, you died 10 yearz ago in ze odder world. How can you say for sure zis iz ze real you? Maybe for you it is evident, but... I wonder if you ever really were Serge...? Furthermore... what waz zis Serge? A figure, a shape? A spirit? A soul? Where waz zis Serge?
For the most part, I think this is Harle just taunting Serge. Since he arrived in Another World, he had been protesting that he was Serge, and in reality, he wasn't. He was a Serge, but in Another World Serge was long dead. Now Harle is making fun of him because he's still saying he's Serge, and she's kind of saying, "but are you really?" Has he ever really been Serge? After all, the real Serge was supposed to die.
I think the "mind control" over Kid must have something to do with this. Serge "becoming Lynx," so to speak, was the first real substance Another World gave him, and maybe he was even beginning to wonder if he was Serge. It's not like anyone in Another World gave him any validation up to that point in the game. In fact, he garners more acceptance from the demi-humans as Lynx. It isn't until near the end of the game that he starts learning more about himself and who he really is. Maybe that's why Harle finally starts calling him "Serge."
Not that a simple mind control spell seems out of Lynx's realm of power, but it's just to easy a solution for me. I like to think of it as, no one really knew Serge. He didn't exist in that world; his companions were following a ghost. So if someone shows up looking like Serge, even he is acting weirder than when they first met him, who are a group of people who just met him to claim that this isn't the person Serge was all along?