The main character of Chrono Cross, Serge follows the Square main character tradition of being young and around 17 years of age. Just like Crono in Chrono Trigger, Serge is totally mute, and won't have much to say obviously. Born in Aruni village, he has been living there all his life. He seems to play a very important role in the story, evident as Yamaneko has been in search for Serge for some time.
Kid can be said as the heroine of Chrono Cross. She actually comes from Lucca's orphanage, but besides that her past seems to be a mystery. Stuff like how she became a thief and why she's wearing such an indecent outfit will likely be revealed during the game. She seems to have a little history with Tukuvomi and Yamaneko as well, apparently tied to her past.
Where you have Serge as a clear good guy and Yamaneko as a clear bad guy, it is hard to put Tukuvomi in either category. She seems to be Yamaneko's little jester, but she doesn't really oppose you. Early on you'll see some kind of rivalry between Tukuvomi and Kid, and when Tukuvomi blows Serge a kiss....well, let's just say Kid ain't happy.