Colt appears throughout various parts of the game to deliver the player's mail, and to tell the player when there are sales in the Market or discounts on drills. She also visits the player when it is the player's current monster's birthday.
When the Four Heavenly Kings are summoned, they take Colt as a hostage. When Colt and Electra are being held as hostages there are no discounts on drills. The player rescues Colt when they win the Naga Coliseum tournament.
Colt's full name is "Coltia", however she is commonly referred to by her nickname "Colt".
In Monster Rancher DS, oddly, even while Colt is being held hostage, she shows up to tell the player when there are sales at the market.
This is likely a programming mistake, as the programmers of the game likely forgot to have Market discounts disabled when Colt is kidnapped.