From the Super Mario Wiki
“Please! Go out with me!”
Lock was forced to work for Mimi in Merlee's Mansion in the game Super Paper Mario. He can be found in Chapter 2-1 of Gloam Valley. If the player talks to him as Peach, he will infatuated with her and will tell Peach it is fated that they met. He will ask her to go out with him, which Peach will reject by telling him that she is going to Merlee's Mansion. He then warns her about bad things that will happen to her if she goes to the mansion. Peach asks him what would happen which she will then find out that he doesn't even want to remember. After that, he once again asks Peach to go out with him. Peach again rejects Lock, but tells him that if she doesn't come back, he can come and save her.
If the player talks to him as Mario, he will tell him to 'get lost' and go to Merlee's Mansion, contradicting what he says to Peach.
If Peach talks to Lock after defeating Mimi in the mansion, Lock will express his jealousy for Mario because he is traveling with a "cute cutie" (Peach). After beating the game, he will remark on how Mario saved the world and will likely attract a lot of girls before asking him what his secret is.