In the manga, Devil Master was a Hidden Pet, which is a rare type of Digital Pet that lives much longer than regular pets and has other superior properties. Such pets are identified by having a star in their graphics.
Devil Master became more like Kujirada's Master and often kept him awake, demanding to be fed. Kujirada began feeding other pets to him, including Peachy. He tried feeding it U2, but using the data it incorporated from Joe, U2 defeated and destroyed Devil Master.
Despite the loss of a powerful Pet, Kujirada was happy that he would at least get some sleep.
In the anime, Devil Master was a Gold Pet, which was a more expensive Digital Pet with superior properties to a regular Digital Pet. Kujirada stole it from a shop.
Kujirada tried making Devil Master stronger in order to win a competition. He did this by swiping other people's pets and feeding them to Devil Master. Among Devil Master's meals were Peachy and Joe. Kujirada tried to feed it U2, but Honda arrived, after spending 3 sleepless nights taking care of Super Strawberry. Honda connected Super Strawberry to Devil Master. Super Strawberry swept Devil Master away.